All fine and dandy IF you assume that Parking Cos are altruistic organisations who never ever exceed their remit, or are never anything other than reasonable in their approach.
I've not personally fallen foul of them but have enough friends and relatives who have to know that even in clear-cut cases of excessive zeal or being plain wrong, they continue to send out threatening letters for months in some cases.
There's also the case that something like a £90 fine for over-staying 5 minutes at an ASDA is out of all proportion to the harm suffered. Sending say a £10 checque in "full and final settlement" and laying out in writing that you believe that this is a reasonable recompense for the inconvenience to the car park owner generally works.