I was about to come here and start my very own thread on this same problem.
I was on the 4 Meg service and about 8 weeks ago the service suddenly feel to pieces and on a good day I was getting perhaps half of what I should have been, about 2meg.
Well they send engineers out to change modems, bits of cable and line test and could... wait for it... nothing wrong.
They agreed I wasn't getting the speed I should, but all the equipment was fine and they were puzzled.
The issue was never fixed.
Now this week they put me up to the 10 Meg service. Yippee I thought, maybe a magic solution would be found.
Uh, no.
During the day I now get less, maybe a 1meg service if I'm lucky. At night I get no more than 5 meg.
Just like the 4 meg service I can only actually get a maximum of half what they promised.
Before 8 weeks ago I had a faultless service literally for years.
Now that has disappeared and Telewest are ABSOLUTELY clueless on how to fix it.