How are you paying, Direct Debit? If so trundle off to your bank and insist they refund you under the Driect Debit Guarantee.
Write to ntl, recorded delivery, enclosing a copy of your agreement. Yes they should have it on file, but we are talking ntl's rather messy systems here. Complain about the overcharging, and state you have sought refund from the bank, as ntl seem incapable of doing so, and that you want correct billing in future.
As for breach of contract, I would suggest yes. ntl are only entitled to bill you for the contrated amount. More than that is wrong, and any refusal to refund immediately is theft. But then I'm not a lwyer, so what do I know
