Yes, I had suspected that the landowners may be concerned about litigation in case of an accident in the quarry. Commonsense dictates that if someone is injured or killed while trespassing on your land, having broken down your fence, then you (the landowner) can't be held responsible. But commonsense and the law are two different things.
A week or two before my acquaintances met the security team in the quarry, the Leics police had posted on social media a list of warnings about the dangers of the quarry lake -- although the post also included the words, "We are not looking to spoil anyone's fun . . .". I am entirely in agreement with this approach; what upsets me is that someone has now decided that the informal use of the quarry should be totally prevented, even after steps have been taken to warn people of the risks.