Cancel direct debit, get in touch with Citizens advice, they may want to you to call down and see them, take any copies of bank statements you have showing payments made ect.. they will fight your corner and give you the advice you need to get this matter sorted..
Reason for cancelling DD is you'll be shocked how soon you get a statement through the door showing your balance and notifying you that you owe a payement..
Good look with this but is seems to be a growing trend now with storecard finance companies...