Seems Wal Mart over here in the US contracts out the store cleaning (floor polishing etc.) and shelf stocking during the graveyard/night shift to local contractors, (think you call them 'gangmasters in the UK) , seems the Immigration have twigged on and now carry out raids during the nightime hours , makes it easy work for them since the stores are in lock down mode , no one allowed in or out to prevent theft, and also good for Wal Mart/Asda's image since none of the public gets to see and be upset at the sight of illegals being bussed away in cuffs
