David Pursehouse, who runs Mills Stores in Doncaster, doesn't know whodunnit, but the 'apparent' customer is a well-turned out geezer who swanned into the store on his mobile talking constantly to her indoors. who is supposedly giving him shopping advice.

"Red or white wine?" he might ask, or "They've got three kinds of milk. Hang on, I'll ask if they do top-ups." He looks as though he can well afford the £15 in groceries and the £50 Vodafone top-up.

David prints out the £50 voucher at which point the man asks for cigarettes, meaning that David has to turn his back to pick them from the gantry. But alarm bells were already ringing because he remembered a case six months earlier when the local petrol station got done in similar sounding way. So he kept the voucher in his hand, turned around, looked the customer straight in the eye and said, "£65 please."

Cool as a cucumber the man says "Oh, hang on, think I might've left my wallet in the car" and walks out saying into the mobile to his imaginary other half, "Did I leave my wallet behind? What, on the coffee table?" And then to David: "Sorry mate, I'll be back in 10 minutes."

He then disappears into thin air.

David wishes to pass on this warning, especially as two other local stores did get caught shortly afterwards by the same tactic.

The mobile phone is actually turned onto camera mode. The con is the photographing of the voucher with its 13 digit number. Then the voucher's worth can be activated or sold on.

The CTN owner just up the road from David didn't realise that she had been conned until two days later when a customer actually purchased a £50 voucher only to learn that it had already been used elsewhere.

"I could've grabbed him," points out David, "but what for? He hadn't committed any crime in my store."

The retailers caught out are now facing a loss. Vodafone won't compensate and if the powers get their way you could eventually have to hide the cigarettes under the counter, so your back will be turned longer. Please pass this on - and watch your backs.