Being an honest-ish sort of chap (and not actually wanting to begin a life of crime for a £4 supermarket bill) I found one of the shop staff and told her, which is where it started to get weird. Because she took it (and me) round to the customer service desk and explained the circumstances, and she was told to take care of me while the customer service person took the card and receipt to the manager's office for safe-keeping.
Taking care of me consisted of scanning the stuff I'd bought, as you might expect, and asking me to touch my card to the machine. At which point it told me I was being refunded the money. I expressed some surprise and she agreed it did sound odd, but it was what she'd been told to do. She then scanned the stuff again, and this time charged me.
She seemed to think it was how things were supposed to work. I really have no idea, but I will be reading my next credit card bill with great interest. Because I've a suspicion I just got £4 worth of groceries for nothing...
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