You'll probably get shunted around between departments but start with 0800 800 150.
Further, any organisation taking money by Direct Debit is obliged to inform the account holder
in advance of each payment being taken. (They can take regular payments of the same amount without notifying you every time but they must let you know if there's a change to a regular payment or an individual, one-off, payment). So you can claim your money back, through your bank, if no such notification was received:
http://www.directdebit.co.uk/DirectDebitExplained/Pages/Benefits.aspx