Afaik, if you haven't informed NTL (in writing) that you no longer want their service then you have 'been in receipt' of that service and they can bill you for it.
What to do now? -
1. Pay the outstanding bill - it will only get worse when they call in the debt agency! Probably best to ring CS and ask for a bill to be sent to your current address and tell them to stop using the DD.
2. Write to NTL confirming that you no longer want their service (and the termination of the DD). They have a 30 day cancellation period, so you WILL be billed for more
3. Cancel the direct debit via your bank.
Moral - NTL are a service provider. They provide that service until YOU say stop.
Thats my view anyway!