I'm with TalkTalk, via Carphone Warehouse from whom I got my mobile. I find the best way to get action with a problem is ALWAYS to go into one of their shops.
I've been into those in various areas inc several in London, Bristol, Subury in Suffolk etc - and I've always found the staff exceptionally helpful, very well trianed, and even willing to stay after hours to get your propblem sorted. They always know exactly who to phone to get a result - it saves your own phone bill too
Having said that, their phone lines are cr*p for using my laptop...