it'll be when you paid it in they keyed your account number in wrong, so the payment still leaves the otherguys account but if the details for you are incorrect they go to a temporary holding account called rejected entries fir your sort code, people check this account on a daily basis and match up the rejected transaction with the reciepts from the counter staff, then sort the transaction out.

This will be whats happenin with your cheque mate, sit tight as this happends soooo often there will be a backlog, hense why the guys above always got his money through a week later

PM me if you wanna know more, I did cheques for 3.5 years with Barclays