Tis an interesting one this.
But one thing I was thinking about was this, "bloke then said 'its no9t a public highway' so maybe same rules don't apply"
However, according to an interpretation of s.192 of the RTA '88 (possibly in a case called Clarke v. Kato, though I've not found the specific extract yet - just a determination that a car park was not classified as a road) a car park is a 'pubic place', and the public enjoy the right of passage over a public place.
So it 'might' be that if something is classified as a car park, then it is public, and you can't stop cycling.
But if it's classified simply as private land.... In that instance the owner can permit or deny access as he sees fit, otherwise someone is committing trespass, and by putting signs up the owner is effectively revoking an implicit permission.
The thing is, though, how do you enforce it? Trespass is, for the most part, a matter of tort - i.e. it's civil, rather than criminal, which means in order to enforce it you'd have to raise a court action to get an injunction to stop that single person from being able to cycle across the car park. Are they really going to do that?
I'd be inclined to FOI what action is to be taken against a cyclist who cycles across the car park, and then bung it in the direction of the Cycling Silk! In fact, wonder if he's amenable to a tweet of this thread (even if I'll likely be told most of the above is wrong - my defence, this is English law, and as has been stated Scots Law is much simpler on this!).
Oh, and the Multrees stooshie. It was a friend of mine basically manhandled out after taking a picture of an interesting shop window display (of shoes). So we organised a sort of photo flashmob. And wound up with more police than mobbers, most of whom were awfully nice middle-class types with big SLR cameras.