Hello, thank you for your question. My name is***** can help with this.
If the machine was malfunctioning, that should be a sufficient ground to say that they are not entitled to charge you for the penalty that they now seek from you.
You have, of course, already paid the sum requested, and was prevented from displaying a ticket by the malfunctioning machine. In other words, you have performed your part of the contract, but they have failed in their is offering the ticket that you would use to display in the vehicle.
You should be able to challenge the charge solely on this basis.
Additionally, if you did receive the landowners consent to leave the vehicle there, in the circumstances, and that would amount to another basis of defence. If either of them was accepted, and there is no right to issue the charge.