Get onto your local council, they will have a consumer protection department that can advise on these issues.
Given that presumably the fault was not present before the telewest tech attended I would certainly take it to the small claims court.
It will cost around £40 or so to get the ball rolling, they will help you with the paper work, if you want to get an independent report done to find out who is at fault you can add the cost of this to the claim I think, check it out first through your local court.
If the tv cannot be repaired, you're looking at replacement costs, put this on the claim too, plus claim for inconvenience, distress etc;
My guess is that once you get the court date through the post you will receive a nice cheque from telewest.
I can understand how you feel, the tech allegedly acknowledged the fault occured while he was there but telewest have now tried to brush it under the carpet, find out your rights and let us know how you get on.
Ooops. forgot to add, if you take the small claims route and the court find in your favour then telewest have to stump up the money you paid to start the proceedings too.