Any history of the vehicle; since, you owned it and has anything worked correctly previously? We have seen a wide range of issues related to interior electrics not working; the wiring harness, flex areas of the doors, from the body to the doors are a known problem. They can eventually break wires and this kills the electrical power to the doors; the harness, is encased, in a ribbed rubber sheath and this can be pushed back, to expose any broken wires.. As these cars age, previous owners or repair shops; sometimes, swap in used parts, in attempt to save money and while, thes parts may "fit", look the same but can cause malfunctions (window motors, comfort control modules, etc), as they may not be compatible. We have found members, who were able to inspect the parts; identify incorrect part numbers or note the markings, that breakers/junk yards use to identify their parts and once, that was found, correct parts sourced, installed, cleared up functional issues.
While, conventional testing of VW electrical systems; can be done, it really helps, to use a factory level scan tool, to view read any trouble codes, confirm the communication bus is up/down, use live data and output testing, to confirm, functionality of various troublesome areas of the interior electrics, you are having issues with. The main issues we see, are defective, incorrect parts and wiring harness problems, killing power to things. A bad part, say a comfort control module, etc; has been known, to bring down the communications bus and that is why things stop working, as the computer cannot command things.
There are many, VW factory level scan tools available; VCDS by Ross Tech and OBDEleven, are the most popular and then, there are the many "VAG" scanners, on sites like Amazon.
A simple attempt to clear things; might be to, remove the positive and negative battery terminals; touch them together and reinstall them on the battery, then see, if any changes are noted in things that are not working. If this helps and then; things stop working again, this can be a sign, that a bad part or problem, is bringing the bus down.
Like, most problems, troubleshooting, comes down to a process of elimination and one, can throw parts at the problem, until you hopefully find the bad part or issue, then you can use a VW factory level scanner, like VCDS, to gain insight, into what is working or not, read trouble codes, view the computer info, that car is "seeing", etc.
These type of repairs, take time, effort attention to detail and can really be helped by using a factory level VW scan tool. Some, of the early convertibles; had specific motors and window regulators; many have noted, that VW revised these parts and in the process, of repairs, had to swap parts over to the later versions, to get things working right. I don't know, if this was the same in the UK or if you could go to a VW dealer and check the history of the car, repairs or updates, upgrades, recalls, that may have been done in the past, etc.
Let us know, how you want to proceed and if you want to attempt "diy" repairs or revisit, a professional repair shop or vw specialist, to sort things out. Thanks.