Just for a start, I'm giving this advice blind as I do not have the same set-up as yourself.
First of all, are you sure that you analogue-out connection is outputting to the tv card?
Secondly, have you made sure that the settings on your set-top box is allowing and analogue output?
Have you tried scanning using the Hauppauge software? If you have, then you may find that it will only pick up one channel (i.e; the channel that your Telewest box is set on. Any channel changes will have to be done through the Telewest box.)
Media Centre is a pain in the bs when it comes to configuration with TV/SAT, but try the above first.