I have and had back background checks carried out prior to starting my apprenticeship. They even sent a a "special policeman" to interview one of the guys who supplied a reference.
We also used to get spot checks at the gate by the Civil Nuclear Constabulary. We also had several hoax bomb threats that had people out of cars and mirrors underneath before entering site.
More recently some travellers set up camp, but they were gone in in less than a morning. Your not going to argue with armed Police.