I cannot claim to be as knowledgeable in this area as lenny01. He certainly seems to have given you some excellent advice that I would hopefully remember if I was in a similar situation. But this thread brings up some very basic principles. The European Convention on Human Rights, backed in the UK by the Human Rights Act 1998) shows that articles 5 and 6 have been breached here. The security guards do not seem to have given the reason for your detention and they have presumed you guilty in the absence of you proving your innocence. As for the giving due notice of 'spot checks', I used the self service checkouts today at Ikea. They had very clear signage, boldly written at every till, that they would carry out such checks, I have never seen it in a supermarket. It all brings to mind a conversation I had with my dad (a retired police officer) about police operating road side checks for drink drivers. These too are illegal if they are only doing it to breathalyze you. On TV shows where these checks are being shown they often tell the driver they were slightly over the speed limit, giving them reason to breathalyze, but if they have no proof of wrongdoing you are entitled to refuse without fear of retribution. Simply ask if and why they suspect you of 'committing a moving traffic offence'. It is the same in this instance, ask the question about what they suspect you have done and the proof thereof, when none is forthcoming make to leave (sounds as if there were plenty of witnesses, if they lay their hands on you its common assault.