Money saving expert has some good advise on this subject:-
https://www.moneysavingexpert.com/reclaim/private-parking-tickets/
If I were you, I’d be willing to take the matter all the way to a court.
I believe that they would loose such a case – since they could not show that they had suffered a loss due to you entering extra digits on your ticket (after all, you paid for your parking).
Of course they could argue that you had not complied with their rules (entering the correct reg), but it is not like you have entered some random car reg which is nowhere like that on the car parked.