It can't be enforced in the way a council or police issued Fixed Penalty Notice can - the ticket is simply an invoice which must be pursued in the civil courts if the recipient refuses to pay.
They would have to prove:
Who was driving
There was a breach of contract - the driver knew he (or she) had to pay
My local park, with fishing lake, has a pay and display car park, unfortunately operated by the council.
There are letter templates on the internet should you wish to challenge such a ticket.
Usually it is too much trouble, and too expensive, for them to chase it up if you refuse to pay