I didn't realise it takes a day or 2 to be debited. I woke up this morning to find I've had bank charges taken for being overdrawn after using tesco pay at pump. I now have the princely sum of £2.30 until Tuesday!
Luckily the cupboards are well stocked but I'd like to warn others who didn't know about this or it could really bugger up someone's finances!
Oh and B&M take 3 days to debit the money 
Surely a debit card means it goes out straight away?