sadly you are not alone its a corruption with the EPG and the channels list from the provider.
Sometime the web EPG provider doesnt have a list for the original transmittor you choose and appears to what it thinks is a suitable alternative. When your MCE downloads the EPG it overwrites the previous one (which worked) with one that also works but now no longer has the correct Channel ID# associated a MCE goes "No Channels Found, please rescan..."
Some fixes are (annoyingly) either Close and reopen MCE or a complete restart sometimes kicks the EPG DB into gear
Or setup a scheduled backup job to copy the C:\Documents and Settings\All Users\Application Data\ehome\EPG (I think its that spelling but you'll see it). Then if it becomes corrupt a simple copy'n'paste will restore it so you can fiddle later rather than in the middle your fav TV channel
I have seen some more permenant fixes on these forums. But the UK EPGs do seem to corrupt MCE on a far to regular basis making the whole experience far from ideal.
BTW I used to have Telewest and loved it, I moved away just before they bought out their 3 tuner PVR, to be honest if I'd stayed on cable I would never have got a MCE!