Twenty years ago, as a struggling and penniless sous chef in Paris, Giorgio Locatelli once saved a month's salary simply to buy a cut of beef. But then it wasn't just any cut. It was Wagyu, and it was making its European debut as the world's most expensive meat. Yesterday he tasted it again, though this time it was free. Which was just as well, really, since the rare ingredient, imported from Japan, was the gourmet filling in Britain's most expensive sandwich - a speciality which goes on sale from tomorrow at the Sandwich Counter at Selfridge's, in Oxford Street....