Handheld devices and mini-computers attached to trolleys and baskets can direct shoppers around stores, inform them what's on special offer, and calculate spend as they go along. Alan Townsend at ­Wincor ­Nixdorf says that at one end of the scale the Waitrose hand-held scanner for self-payment is the most basic kind of PDA. This recognises a loyalty card and totals up a customer's bill as they scan purchases. At the other end of the scale, the rubberised tablet laptop used by member shoppers in Metro's Future Store is highly sophisticated. It can pull up a user's typical shopping list, direct them around the store and show details of promotions.

"It would be too costly for fast-turnover superstores to offer customers the Metro-style PDAs, but in the future there will be something in between," says Townsend.