PEA/Smart fines etc aren't worth the paper they're written on. Bin immediately, usually. Same goes for the clamps. If you leave it for a couple of days they know they're not going to get paid so remove it to place on some other poor sod's car. The fact this was a rental fine though changes things. Presumably the fine went to them, and they're hounding you for it? I can't imagine any rental company is going to want this hanging over them, whether they know the PEA are a load of criminals or not.