TITLE A DUVET COVER. FIELD This invention relates to textile products, to bedding, and in particular to covers that hold 5 duvets or can be used as bedding on their own. DEFINITIONS and SYNONYMS A "duvet" is a soft flat bag traditionally filled with down or feathers (originally the down feathers of the Eider duck) and used as an insulating cover over sleeping persons. Duvets 10 are believed to have originated in Scandinavia. Synonyms: "quilt", "continental quilt" and "doona". (Doona is a genericised trade marked name.) A "comforter" (US usage) is functionally the same as a duvet; the insulating filling including polyester batting, down feathers, wool or silk. A "duvet cover" is a soft flat bag which holds a duvet. The duvet cover, without an 15 enclosed duvet may alternatively be used as a less effective insulating cover over sleeping persons in warmer weather. Hence the bag comes into contact with the persons in the bed. For a zip fastener, we use the term "zipper" to refer to the metal travelling item which brings the tracks together or separates them. 20 BACKGROUND Duvets have become popular in the last 20-30 years in the Western world as a warm yet light-weight substitute for blankets. A duvet comprises a generally rectangular thick layer of insulating material, confined inside a cover made of a textile. It is preferably sufficiently wide and long to cover a bed that it is designed for, whether the bed is a 25 single bed, double bed, king-size, queen-size or super king-size - or other popular names for various sizes in a particular culture. The duvet is kept inside a duvet cover which can be washed and dried and replaced over the duvet. However stuffing a duvet into a cover has been found to be difficult to perform. In the known prior art, Kraan (W02006/011794 describes an example of apparatus and a 30 method for putting a duvet inside its cover. That there has been an invention created for this purpose supports the present inventor's experience that people have trouble in managing the process of putting a duvet inside its cover without any creases, folds, mis orientation and the like, including getting the comers of the cover filled. Elderly people may find this task impossible. No prior art is known in which a duvet cover has fasteners, 35 particularly zip fasteners, such that a top layer comes completely apart from a bottom layer. OBJECT The object of this invention may be stated as to provide an improved duvet cover, or at least to provide the public with a useful choice. 40 STATEMENT OF INVENTION In a first broad aspect this invention provides a duvet cover having a first surface which serves when in use as a decorative, upper surface, and a second surface which serves when in use as a lower, body contact surface, each comprised of an appropriate textile fabric; the length being sufficient to cover the length of a bed, and preferably the width 45 being sufficient to cover the width of a bed upon which the cover is to be used; wherein the entire perimeter of each surface is provided with a reversible closing apparatus so that the first surface may be joined to the second surface by means of the reversible closing apparatus thereby creating a cover with an enclosed cavity capable of holding a duvet, yet 2 from time to time the surfaces may be separated for the purpose of receiving a duvet, and 50 for the purpose of cleaning or laundering. Preferably, the reversible closing apparatus is a zip fastener extending around the entire perimeter of both surfaces; a zipper being provided in order to join or separate the individual complementary parts of the zip fastener. Preferably, the zipper head and tail are placed at one foot comer within a pocket formed 55 so as to conceal the feel of said apparatus from the body of an occupant. Optionally, the first surface further includes a layer having thermal insulating properties of lower effectiveness than the insulating properties of a duvet, so that the duvet cover can serve as a complete cover over a sleeping person in relatively warm weather. In a further option, the layer having thermal insulating properties is divided along the 60 length of the duvet such that a first side of the duvet cover provides greater thermal insulation than the second side of the duvet cover so that, when in use, the duvet cover provides for different perceptions of heat or cold by different sleeping persons. Preferably the second surface is made of a textile fabric selected from a range including cotton, polyester/cotton, flannelette, percale, and "Minklon"; the fabric being capable of 65 being used in contact with the skin and tolerant of repeated laundering. Preferably the second surface is provided with a flap along and extended from an edge corresponding to the head of the bed; the flap being joined to the adjacent side edges of the second surface over the length of the flap and being retractable so that, when in use, the flap is concealed beneath the cover as when the bed is in a daytime or "made-up" 70 mode, or is folded out from under the cover and use as a pocket, protecting the adjacent edge of the first surface from being soiled when the bed is in a night-time or "occupied" mode. 3 In an alternative aspect the assembly of a first and a second surface provides a repeatedly 75 re-openable container for a duvet; wherein the reversible closing apparatus may be closed around the duvet so as to comprise an effective cover for the duvet; the first surface provides a decorated appearance above the duvet, and the second surface beneath the duvet provides a sheet that may be used as an overlying sheet over a sleeping person and may be removed and laundered from time to time. 80 In a second broad aspect a method for enclosing a duvet in a cover according to claim 1: the method comprising the steps of (1) placing the second surface on top of an existing horizontal surface such as a bed, (2) placing the duvet on top of the second surface, (3) placing the first surface on top of the duvet and with the edges of the first surface in approximate alignment with the edges of the second surface, and (4) joining the first 85 surface to the second surface along their entire edges by means of the reversible closing apparatus. Optionally the duvet can be left out, so that the cover (comprising the first surface jointed around its edges to the edges of the second surface) is provided as a lighter-weight form of bedding. 90 PREFERRED EMBODIMENT The description of the invention to be provided herein is given purely by way of example and is not to be taken in any way as limiting the scope or extent of the invention. Throughout this specification unless the text requires otherwise, the word "comprise" and variations such as "comprising" or "comprises" will be understood to imply the inclusion 95 of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. DRAWINGS Fig 1: is a plan diagram showing the overall structure of a duvet cover according to the 4 invention. 100 Fig 2: is a diagrammatic section through A-A showing the duvet cover when including a duvet. Fig 3: is a diagrammatic section through A-A showing the duvet cover when not including a duvet; yet including an optional small amount of fill or insulation as part of the first surface. Please note that a further option deletes the insulation 105 layer, 101A. Fig 4: shows ancillary details of the optional pocket formed at the head end of the lower duvet cover. EXAMPLE 1 110 As shown in Fig 1, this invention 100 provides a duvet cover having an upper, exposed adorned or decorative surface 101) and a second lower, body-contact surface 102 of substantially the same length and width. The lower sheet may, for protection of the upper cover, have an extension flap (see later). The lower surface normally serves the purpose of a top bed sheet and comes into contact with the people in the bed, when in use, and so 115 should be washed weekly or after each use (for a hotel) or as otherwise required. Each surface is comprised of a selected textile (see below). The entire perimeter of each surface (103, 104, 105, 106) is provided with a detachable zipper 108 right around the edge, so that the first surface may be joined to the second surface at the corresponding edges by means of the reversible closing apparatus. Other reversible closing apparatus (such as 120 domes, buttons, cord with apertures, or the like might be used instead.) When joined together, the duvet cover has an enclosed cavity (see the section A-A in Fig 2) capable of holding a duvet 109 although as described later there may be seasons in which the duvet is not used; just the duvet cover is used. The zipper is preferably located substantially right at the edge of the sheets of material 125 comprising the duvet cover. Trials so far that seek to conceal the zipper by aligning it inside the perimeter have not been rewarding. 5 From time to time the surfaces 101 and 102 may be detached from each other thereby allowing a person to separately clean or launder either or both surfaces. One might expect the second (sheet) surface 102 to become dirty in about a week of use, especially if 102 is 130 the only sheet in contact with the person or persons using the bed. It is difficult to launder that second sheet unless it can be detached from the first sheet, since the first sheet has more bulk and is likely to include thermal insulation material that is difficult to dry. Some prior art is known in which the first and second sheets are permanently joined like an oyster shell at its hinge, or like a book. Such examples could not be laundered separately. 135 The length of a selected duvet cover 100 is sufficient to cover the length of a bed (which may for example be 83 inches (211 unu), and preferably the width of the selected duvet cover is sufficient to cover the width of a bed upon which the cover is to be used. There is a variety of bed sizes in common use, such as: single bed, double bed, king-size, queen size or super king-size - or other popular names for various sizes in a particular culture. 140 Typically the first or upper surface 101 is made of a fabric having a decorative appearance so that a user can leave the first surface exposed when the bed is not in use for sleeping. Example fabrics include cotton or polyester or blends thereof, and may include wool, silk or any other popular textile. Colours may be varied according to fashion and demand. Patterns may be created by use of colours, embroidery, applied 145 textures and the like. Beads and reflective materials may be used, since this surface is laundered only occasionally. Quilted surfaces may be used. As a preferred though by no means essential option, the upper or first surface 101 is provided with an extra internal layer 101A comprising a fill having a thermal insulation property. The upper surface may be obviously quilted in order to hold the fill in place, or 150 the fill may be held in place with a fastener in each corner. One preferred fill is a polyester fibrous insulation fill at a density of from for example about 120 g/square metre to about 300 g/square metre, so that the duvet cover 100 (comprising the first and the second surface) can serve as a light-weight thermal cover and sheet for sleeping purposes. For instance the insulation merit ("R" value) of the upper surface may be perhaps one 6 155 eighth to two thirds of the merit of a duvet itself, so that the duvet cover of the invention can be used as a less effective thermal cover, somewhere between the insulation quality of a sheet alone, and that of a duvet. Optionally the upper or first surface 101 can be provided as two halves - split lengthways. The main application for this is in a double bed, where it is often stated that one person 160 feels hot although at the same time the other person feels cold. This effect, which may reflect differing metabolic rates, is often found to be a cause of difference between couples; what is too hot for one is too cold for the other. Accordingly, the duvet cover of this invention provides for different perceptions of heat or cold by different sleeping persons. That extra internal layer comprising a fill having a thermal insulation property 165 may be supplied in two separate parts, each having a similar length and width but with significantly dissimilar insulation properties. Each part may be held in position within the duvet cover by use of attachment means in four corners; the underside of the upper layer of the duvet cover having eight complementary covers. Preferably the lower or second surface 102 (obscured) which serves as a sheet is made of 170 a woven cotton or a polyester-cotton blend, a flannelette sheet, or an equivalent, and preferably tolerates repeated laundering. Contact with this surface should feel nice to an occupant (or occupants as the case may be) since this surface is likely to be in contact with the occupant for extended periods. This sheet might have a thread count of at least 100 per inch; more preferably about 300 threads per inch (120 threads per cm). The 175 textile known as "percale" (a closely woven plain-weave fabric) is an example that is suitable for use as the second surface. It may be dyed as appropriate, such as to match or be complementary to the colours of the top or first surface 101. The false fur material called "Minklon" is a further option; said to have a sensual feel. The preferred reversible closing apparatus is a zip fastener (or zipper) 108 extending all 180 the way around the circumference of both surfaces. The tape ends of the two complementary toothed parts would be sewn together, end to end, or preferably run past each other (as at 107) so that them is an overlap. More preferably the ends are secured 7 inside a small triangular pocket 111. The type of zip fastener (open-ended type) that allows complete separation of the two parts 101 and 102 is desirable so that the duvet 185 cover can be completely separated and each half can be cleaned separately as and when required. A good quality "YKK" brand zip (or equivalent) of sufficient length is sewn around the circumference using techniques known to those skilled in the trade such as oversewing to better secure the textiles to the zipper sides. The colour of the fabric tapes and of the teeth of the zipper used can be selected so as to merge with the colour of the 190 surrounding textiles, or an invisible or concealed zipper can be used. A single long zipper 108 is preferred, if only because there is then only one zip slider handle and ends to be concealed, and because it is more convenient to separate or rejoin the parts of the duvet cover in one action. Alternatively the invention may be made with more than one zipper, which may help if zippers need to be replaced. The spirit of the invention resides more in 195 the concept of providing complete separation of the first 101 and second 102 surfaces (such as for laundering from time to time) than it does in any requirement for a single zipper or indeed any one fastening means. Preferably the single zipper slider and box are placed so as to lie at one foot corner of the duvet cover, and optionally a pocket is formed around them (at 111) in the material of the 200 second surface so as to prevent contact between the zipper parts and the toes of an occupant. Reverting to the primary function of this invention as a duvet cover, the co-joining of a first and a second surface provides a repeatedly re-openable container for a duvet 109. The reversible closing apparatus (typically a zip fastener) may be closed around the duvet 205 so as to comprise an effective cover for the duvet; the first surface 101 provides a decorated appearance above the duvet, and the second surface beneath the duvet serves as a sheet that may be used as an overlying sheet over a sleeping person. A useful advantage of this invention is that it becomes very easy to put a duvet inside the matched duvet cover. This has hitherto been rather difficult and hinders adoption of the 210 duvet form of bedding. The method comprises the steps of 8 (1) placing the second surface (the sheet 102) on top of a surface which is likely to be the bed on which the duvet cover is to be used (but may be another surface), with the zipper track facing upwards around the flap (if that option is used). (2) placing the duvet 109 on top of the second surface, 215 (3) placing the first surface 101 (the decorative textile) on top of the duvet and with all the edges of the first surface in approximate alignment with the edges of the second surface, and (4) joining the first surface to the second surface along their entire edges by means of the reversible closing apparatus. In the case of a zipper, the box of one half and the free end 220 of the other half are brought together with the slider adjacent the box, and then the slider, moved by the handle, is zipped all the way around so as to enclose the duvet inside the duvet cover. As previously stated, in warmer weather, the duvet can be left out, so that the cover (comprising the first surface and whatever weight of fill is included as part of 101 (see 225 101A in Fig 3) is joined around its edges to the edges of the second surface) hence providing a lighter-weight, two-layered form of bedding. VARIATIONS An additional aspect of the invention is that the second (sheet) surface is optionally 230 provided with a pocket formed by a flap extended from the edge that will be next to the occupier's head and shoulders. Sec Fig 4. This flap 110 is preferably joined to the adjacent side edges over the length of the flap (which is typically 20 cm), so that, when in use, the flap can be either folded one way and concealed beneath the cover as when the bed is in a daytime or "made-up" mode, or folded out from under the cover and placed over the near 235 pillow edge of the top surface, like a pocket, when the bed is in a night-time or 9 "occupied" mode; thereby protecting the cover from wear and providing a better "user feel" around the head or pillow end of the bed when occupied, just like a tumed-down sheet. The zipper track is at the perimeter of the lower sheet but the power sheet has a fold supporting the track. 240 Although the inventor has referred to two different surfaces being used as the first and second surface, both surfaces could be the same. In that case the ease of enclosure of a duvet is retained but the appearance of the article may be diminished. INDUSTRIAL APPLICABILITY and ADVANTAGES 245 The principal advantages of this invention include that: It is very much easier to cover and contain a duvet with this duvet cover. The duvet cover of the invention allows more flexibility of modes - it is useful on its own (that is, without an enclosed duvet) in warmer weather. The sheet side (the second surface) of this duvet cover can be taken off and laundered 250 without the problems associated with laundering a polyester fill (or down) because the two parts can be completely separated. Hence the invention promotes hygiene. A purchaser may own several instances of the lower sheet. A bed is made more presentable - with the decorative upper cover, when not actually occupied. 255 For hotel or hospital use in particular, maintenance of hygiene between one occupier and the next is greatly facilitated. Finally, it will be understood that the scope of this invention as described by way of example and/or illustrated herein is not limited to the specified embodiments. Those of 10 skill will appreciate that various modifications, additions, known equivalents, and 260 substitutions are possible without departing from the scope and spirit of the invention as set forth in the following claims. 11