In order to set up a direct debit they would have needed a sort code and bank account number. Did you give this to them? If you didn't then they must have took down the details without asking, as some debit cards have these details on them and set the DD up that way (fraud?!). If they did set up a DD i am pretty sure they have to run through the whole DD guarantee, about your rights to cancel etc. If you definately didn't give them the sort code and account number and these details were not embossed on your debit card then the payment must have been a recurring card payment. From what i have heard from others, these can be much harder to cancel, as unless you cancel the card entirely (i.e. not just get a replacement with the same card number) they can continue to process payments and there is not an easy way to block them as there is with DD's.