Hi there,
similar situation to yours a few years back. Very frustrating and dispite us emailing in copies of all letters sent by post as back up, they insisted we owed them money.
We explained the situation to the collections company who contacted us on their behalf for 1 months bill. They referred the matter back to NTL and finally we were contacted by a direct contact in NTL. We had to re-explain everything for the umteenth time, they apoliged and it got sorted. The whole thing went on for months, but I refused to just pay it and forget it as they were so appauling.
In our case, NTL send us our regular invoice one month which we paid. Then a few days before the end of the month, we got another final demand invoice for 3cents! where they threatened to cut us off if we didn't pay it. My husband was so furious, he wrote a letter and cellotaped a 5cent coin to it telling them to keep the change. A week later, the feckers cut us off. This was 2 days into the new month and they wanted us to pay for a full month fee even though we had 2 days usage before they cut us off! They told us they would refund us for the unused 28 days of the month, but we had zero confidence in them at that stage as you can image, so refused.
good luck
M