The rhythmic bleep at the tills charts a steady trickle of shoppers flowing through the checkout area at Asda in Ystalyfera, south Wales. It is a typical mid-morning at the supermarket in this former industrial town, 12 miles north of Swansea. Young mothers browse the knitwear section at George by Asda; a couple of older men in paint-speckled work overalls scour the sandwich section for their lunch. It looks much like every other Asda in the country. But this site…