There have been developments (this may take a while, but I do need help)! I eventually got a Kenwood DNX8160DABS system, as it seemed to do everything, and had lots of recommendations. Unfortunately, the local "specialist" who fitted it completely screwed up: he competely ignored the special instructions relating to VWs (the power connections are apparently reversed), and he decided to miss out part of the wiring loom "because it wasn't necessary and there wasn't enough room". The special treatment for VWs is apparently because there is no "A" position on the ignition switch, so you can only turn the radio on by also activating all the dashboard lights etc. I still don't understand why the original radio was fine!
Anyway, after he finally reconnected the power wires correctly, the unit kept packing up, with the message "start up error". I lost so much faith in the local installer (and told VW to stop recommending him!) that I contacted the original suppliers; Car Audio Security in Hayes, and they actually agreed to replace the unit entirely. They drew my attention to the missing part of the loom, and said it was this which was causing the "start up error". They really shouldn't have replaced such an expensive unit, knowing that it was the original installation that was at fault. They warned me that the error was likely to happen again, and - sure enough - it does it quite regularly. I can fix it by removing the unit, and pulling -out and replacing the main plug. I've lost the 30-minute cutout, and have to switch the unit off manually when I leave the vehicle (presumably, since the power bypasses the ignition system).
Car Audio security said I could take it back any time, and they would correct any ongoing problems, and set the whole thing up the way I want it (but I have to pay this time!). Before I do that, I'd be interested to hear if anyone else knows about these radio connection issues on the older T5s, and whether you think I really can re-install the unit so that I can switch it on and off manually (without all the dashboard lights coming on) as well as via the ignition key, and build in a 30-minut cutoff.
PHEW!!