I see no problem with the general contract law. I've never read one but I imagine the signboard in the car park sets out terms and conditions. That's the offer. By using the car park you accept the offer. As long as you stay within the terms then no problem. The moment you go outside the terms - by overstaying - then you are in breach and may be liable for a charge. All that seems pretty clear to me. Where it gets nonsense is when they try to charge you a hundred quid or something for an hours parking. That doesn't stack up and there's no way i'd pay it. I'd be interested to know how many such cases get to court - I'll bet it's very, very few.