If you accepted £7 18s and 6d I would say never mind clumsy bastard what about stupid bastard. 1) It's not enough. 2) It's no longer legal tender.
More likely it was the old: pick 2 out of 3 routine:
They asked for good and fast so they can possibly have gotten cheap!
It was probably £71.86 plus 20% vat. Call it a round ton and we'll forget about the receipt.