The "set up a new DD letter" shows part of the ntl systems work then
Your're best bet is definitely to put this cancellation, and claim that you are being charged for services you didn't want, in writing, snail mail recorded delivery.
The big risk is that whatever happens there will remain a shortfall between what you think you should pay, based on the instructions that didn't get followed, and for which there is now probably no proof that you issued them, and what ntl's systems will say you owe based on teh service levels you did receive. Be careful this doesn't end up a bad debt that affects your credit rating.