Had one of these PCNs 1 year ago. They wanted £100 for overstaying by 20 minutes or £65 if I paid within 14 days. Guess what, if you challenge it and ask them to go to the POPLA (Parking on Private Land) adjudicator it cost them £35 - hence the difference in paying early.
I won and I didn't pay. I argued it is not reasonable or fair under English Contract Law and they cannot demonstrate the loss incurred was anything like £100.
I was actually hoping to go to court but didn't get the chance and probably wouldn't because they know they cannot win and don't want their contract with the Landowner under scrutiny. They rely on people being ignorant of the law and if only 2/5 pay up they are quids in.
Lots of material on the interwebs but my advice is don't pay and argue Contract law etc. There is the option of ignoring their letters but I don't want debt collectors turning up at my door.