It’s an invitation to pay them some money. Nothing more, nothing less. I’ll admit they’re getting slighty smart. £42 is almost not worth doing the legwork required to end up paying nothing. That’s what they’re counting on, and the ones who decide to pay up are the ones who allow this industry to prosper.
Don’t enter into any correspondence other than to appeal their “charge”. When they refuse, take it to POPLA. It’s free for you, but costs them money.
Take photos of signage in the car park. It’s especially useful if the signage is sufficiently vague in its terms but also has wording to the effect that the sign itself is the contract you have allegedly broken.
Ask how they know you parked for so long. If it’s ANPR-style cameras at the entrance/exit, there is case history to show that this means nothing other than you entered/left the carpark. You could have driven round and round for 256 minutes then left, which would entail exactly zero parking.
Ask for copies of their contract with the landowner. They have no legal right to claim damages from you (which is what this invoice is) unless there is a very specific contract between themselves and the landowner. Not a shop on the site, the landowner.
Ask how they break down this apparent loss they’ve suffered and for which you are now being invited to mitigate. A lot of parking companies add in cost of parking attendant uniforms, cost of insurance, all that shite. That is an operating cost, an overhead, not a loss. Some PPC apologists might point you in the direction of the court’s musings in Dunlop Pneumatic Tyre Co Ltd v New Garage & Motor Co Ltd with regards to estimation of loss. Interesting as it may be, the specifics of that court case bear little resemblance to “overstaying” in a free carpark. The people who might point you in that direction will most definitely have quoted only the very specific part that seems vaguely relevant to a parking contract, and not the rest which most certainly doesn’t.
Basically, peppipoo.com is the place to go, even if just for template letters to get the swines off your back.