Help me please direct debit?

i have a student membership with dw sports gym (aka jjb) and i cant afford the payment for the 1st of next month, they said they still need the 20pound payment for the start of next month, (because it has a 10 days notice thing) if i cancle the direct debit with the bank will they not talk money out? or will they take it from the bank in others words i owe the bank 20 pound? help please i use a visa debit (i have no credit cards ect)