chrisjj, you say you paid the bill on the 29th December 2006 - may I ask how you paid.
If you paid by cheque in the post, as it was the last working day of 2006, NTL probably did not have receive the cheque until the 3rd/5th January (Wednesday/Friday), depending if you sent it 1st/2nd class. On the back of my NTL bill, it states "please allow 5 working days for your cheque to be credited", which takes it up to 10th/12th January, for a bill you received on the 1st December.
Did the January bill have your payment on it?
btw, you may find this link useful - same thing with Telewest
Linky