Contract Cancelled but Autopage still Debit me

I telephoned Autopage service desk on 01/08/2015 to give notice to cancel my Data contract at the end of the 24 months. I was informed that the service would be terminated on the 16/09/2015 which was the end of the contract and was given a reference number for the cancellation. On the 23/09/2015 I received a invoice from Autopage billing me for the month 17/08/2015 to 16/10/2015 after the date that the contract termination. I emailed Autopage and informed them that this contract had been cancelled and the invoice that they had generated should be withdrawn but on the 30th September 2015 the still debited my bank account. All communication with Autopage since requesting a refund has been a waste of time. I would advise any clients of Autopage that when they cancel their contracts in future to personally put a stop order on any future payments at their Bank as Autopage cannot be trusted to cancel the debit order.