Our solution was more bizarre. We discovered that even though the item was out of stock, it would let us order one and pay for it before it arrived. Armed with the till receipt, I had what I wanted wanted, proof I had paid for it and the shop was no worse off, still having one item awry in their inventory.

We did consider 'buying' a completely different item that happened to be the same price, but we weren't sure of the exact legal status of that and it would have put the inventory into an even worse state ...