Payment by DD is fully automated. Once it has been input, there is no further human intervention until or unless it changes.
Payment by cheque involves human interaction to open, read, credit and pass the cheque for processing by the bank. The people employed to do this would not be employed if nobody paid by cheque. They have a significant cost to NTL. NTL's bank make a transaction scharge per cheque.
I have never attempted to pay by card over the 'phone, but I wiuld assume that at some point you talk to a human being. As above, these people would not be employed if nobody paid by this method. There are transaction charges levied by the banks for card payments.
So the extra cost of not using DD are significantly more than the transaction charges levied by the banks.