By Law the maximum private parking firms can issue is now £50. If you have been issued a penalty higher than this I would apeal. If its only £50 I would pay it and save yourself the stress and heartache of trying to get it cancelled unless you have good reason to.
There is a 10min grace period before a PCN could be issued, that was extended from Highways and council parking to now include private parking firms.
https://www.moneysavingexpert.com/news/2022/02/private-car-parking-fines-cap-to-be-reduced-by-half-from-p100-to/ also avaible on gov.uk website.
Private parking fines on private land are not real fines, rather an invoice and not legally enforecable but…..
https://moneynerd.co.uk/parking-fines-private-land/ Interesting site, full of useful information but, depends how far you want to go!