Basically yes, if you posted a sign on your private house, stating that a certain standard of behavior must be observed, with a penalty if such a standard is not maintained. You could also issue a notice for claim of the penalty. Consent makes the unlawful, lawful. The statutes state that they can Chase the owner. So they do...I know that by registering a vehicle you are no longer the owner, thats one Avenue to explore, and on a personal note. I would tell them to go...... themselves. Buti couldn't advise anyone to do that....

Could post a clear pic of the "prominently" displayed terms_and_conditions. It looks like there was a condition that you display a valid permit. If it was private land, basically, you are in a contract. Sorry to be blunt... There are a whole range of ways to deal with this. How far down the rabbit hole you want to go vs paying 90 quid is a decision only you can make. There are many "beat the ticket" websites out there. With lots of valuable information and end user experiences. But be warned, apply critical thinking to any advice given, a lot will be untested, and you will get resistance and threats at every turn. But that is actually a positive thing. The louder they scream at you, the more you are succeeding. Good luck, post any results back here please...