Between a sooty power station and a brown canal on the edge of a small Midlands town, there is a long blue building that looks like a smear of summer sky on the damp industrial landscape.
Inside, hundreds of people in orange vests are pushing trolleys around a space the size of nine football pitches, glancing at the screens of their hand-held satnav computers for directions on where to walk next and what to pick up when they get there.
They do not dawdle — the devices in their hands are also measuring their productivity. They might each walk between seven and 15 miles today.
Before they can go home at the end of their eight-hour shift, or go to the canteen for their 30-minute break, they must walk through a set of airport-style security scanners to prove they are not stealing anything.