Just out of curiousity, what set-top box have you got?
There is a longstanding history of Pace 1000 and 2000 boxes doing this. Aparently what happens is that the video decoder crashes, and is restarted immediately by the OS. When it does this, it doesn't reapply the user settings, and defaults to 4:3 CCO mode.
With luck it will be sorted in the next Telewest software release. In the mean time, a solution is to do something that causes the settings to be reapplied -- either rebooting, or using interactive should do this. It may even be enough just to bring up the home screen on Telewest, I'm not sure.
Welcome to the site, by the way