By which time it will have escalated through debt recovery companies, and the amount you "owe" will have increased to several hundred pounds.
Factor in their legal costs should you lose, and yours is a bit of a high-risk strategy. A low-key one of approaching the business whose car park it "is" (assuming you were a customer of theirs whilst charging) asking them to ask the parking management company to cancel the charge would be safer.
I know what you're saying, but whenever I have tried that, the business has told me that "It's completely out of my hands, mate because the parking company has nothing to do with us."
I have had two cases now where Mrs S had permission from the businesses to park free of charge, yet we were sent £100 invoices and both went to debt collectors.
One was a fast food joint where Mrs S was working with a young person. The other was a hotel where she was giving blood and the hotel staff told her that the parking had been suspended for the hours of blood donation. In both cases, the parking companies were notified of her reg number.
What boils my p*ss is that, even when exempted from parking restrictions, they still tried it on, knowing she was exempted. And they insisted, even when the situation was explained to them, that they were going to proceed with demanding payment. And when correspondence was entered into, they insisted on sending the "debt" to debt recovery agents, even when the "debt" was in dispute. By the codes they are supposed to abide by, they are meant to suspend any action while there is a dispute.
In other words, they know damn well that there are legitimate exceptions, but hound drivers for payment regardless. Because they know many people will cave and pay up, especially if the magic words "Debt Collection Agency" are uttered.
Disgusting behaviour.
So after a few letters back and forth where I am telling them we are not liable to pay, and them telling us we are and then sending in the DCA, I gave up and sent them a very simply worded letter:
Dear Extortionist
Regarding case XYZ123.
I am more than happy for you to set this case before a judge. I will then present my evidence to them, and they can decide whether the debt you are claiming still stands.
As I said, I have done this twice now and we never heard from them again.
Absolutely hate them with a vengeance.