having disagreement with NTL.

They decided suddenly that they had been giving us double discount accidently for several months, and that when our Direct Debit payment went out, they took the whole of the extra amount (£55.68 -doesnt seem much, but is to us) without notifying us first. When I phoned up, girl who couldnt care less said they were entitled to money back as we had wrongly been discounted twice for something. Pointed out their mistake not ours and why had they not written to us informing us of mistake and offer to take so much extra per month for few months instead of all in one go. Girl said it stated amount to be taken on latest bill, which we hadnt received. Demanded copy, when finally came all it had was normal charges plus amount added on at end, BUT with no explanation!!. have sent complaint to NTL, had one reply saying looking into it & would get back to us within 5 days (but this was 10 days ago!)
Can anyone help? i.e are we entitled to demand money back and repay gradually, and can we do same as with bank charges situations, and justifiably take action. be grateful if any of you can throw some light on this !?!