We recently had an installation by Sustain via Scottish Power. The parts of the work handled by Sustain all went smoothly, with the initial contact person available for questions via Zoom, paperwork and applications seemed to be filed promptly with relevant bodies, and all contractors turning up when expected, with follow up calls after the installation to explain next steps and how we go about setting up a new tariff.
We had 1 teething problem where some of the solar monitoring information wasn't being correctly sent to the cloud, and this was fixed by their support desk within 1 or 2 days with a firmware update done remotely.
No complaints about the service received.