Hi, I'm new as well, and was trawling through the forum posts as I have exactly the same problem! It all started when we moved house 2 years ago, we suddenly started getting bills for "Non-dd analogue set-top box" or some such nonsense. We've never used NTL for TV, we've only ever had broadband and we've always paid by DD. As you did, we called and they kept saying to ignore it, but the bills kept coming. We kept ringing, and kept getting told it would be sorted. We never got any reminders, or threats of disconnection or debt recovery, so eventually we decided to ignore the bills. BIG MISTAKE!
We are currently applying for a mortgage, and have been told we have adverse credit
Why? Because of NTL's screw-up! My partner has been on the phone to them, he asked to speak to a manager and was told that was "not possible" (why?) She did say that she had removed the outstanding balance from our account and that it would be corrected with the credit reference agency.
We have explained the situation to our mortgage company, and they said we need to get a letter from NTL explaining it was their mistake etc. When my partner asked for this, the lady told him we would have to put it in writing to the first. So we have to send a letter to them asking them to send us a letter. We've put together a letter today, will let you know how we get on!