usually sat at the back

The online system does prevent you from paying off more than the balance, however, ignoring your statement balance for a minute, you can pay off your current balance at any point in the month (ie. spend since last statement). I don't see why you could not for example order some stuff from M&S or JL, wait for it to hit your statement, pay off your interim balance and then return the items for a refund. I'd have thought that would result in a credit balance left on the card?

Edit: Just checked my account online, whereas I have paid off the statement in full last week so the payment due is nil - it does show I can pay off the £1000 ish spent since then today if I want to.