I too have repeatedly suffered from the NTL overcharge. To be honest I think their bills are far too complicated to understand if you have multiple services.
I am currently in the process of writing to their CEO with a bill for my time and interest I am charging for them 'borrowing' my money, as I am also sick of
1) being overcharged
2) having to argue with at least half a dozen people to get my money back - persistance does pay off though, Just keep saying I don't want a credit I want my money back, it is my money, you overcharged me, you have to refund it, if they say they can't then tell them to put you through to someone who can!
3) waiting several weeks for the refund to arrive
hopefully I will get my 'bill' paid - we'll see........