BSM
Keith W made a valid point, not just to you, but to the original poster, who has already mention proving in front of a Magistrate.
As far as prosecution of the clamper is concerned, it probably wouldn't get as far as the CPS even if there were evidence of wrong doing on the part of the clamper, and clearly here, there's not.
So he's right to make his point, if for no other reason than to make sure the original poster is aware that parking on private property is NOT a criminal offence, its a civil matter, only persuable through a civil or small claims court.
In fact having read what you'd written, you gave the impression you were talking about the parking 'offence' not any infringement of the SIA license.
Anyway, moving on, a couple of points that are equally important. Supergluing a sign to a windscreen, is probably criminal damage, I'd not do it personally.
Lifting a car off private property an 'dumping' it back on a private road. Silly idea, its not the driver or owner who'd be liable for either the decriminalized parking offence that would follow, or the criminal offence of obstruction, it's the person who put it there! Not to mention, again, criminal damage probably.
While I am generally against the idea of clamper's, unfortunately, in cases like this, there is little option, the civil claims route is a non starter, unless the law changes, so when it is banned like it is in Scotland, It will leave people like the OP in an impossible situation.