Look for signage at the site where you parked. Unless it's very clear and prominent, and there's no reasonable way you could enter and park without seeing it, they have no chance of enforcing. These things are basically done under contract law, so they have to show that you've "agreed" to the contract by parking.
I got one years ago in the hart hill services car park. Binned the ticket, got a letter through the post. Binned that too and heard nothing again.