If the car park company asked for a fiver they may have a case for the moral high ground. As they've asked for £90 they haven't and I would take the legal option of ignoring it.
As I understand it, no-one in this thread has challenged the validity of penalty charge notices issued by local councils. Here's what my local council has to say on the matter:
http://www.chiltern.gov.uk/site/scripts/documents_info.php?categoryID=1358&documentID=284
You will notice that they refer to a penalty charge of £70. The all-day rate for parking in the local station carpark is just over a fiver. The difference between £70 and £90 is neither here nor there - if smutchin's local M&S is in a large town rather than my small town that alone would explain the difference.
The council's rubric also refers to the Traffic Management Act 2004. And that's why I think the
moral thing to do is to pay parking penalties if you incur them. Rationing of parking is one way of managing traffic. If you believe in the appropriateness of trying to manage traffic (and my sense is that this forum generally does) then you should also accept the appropriateness of charging penalties for overstaying parking. It so happens that we've got a real mess of a situation at the moment, where some carparks are in private hands and some in local government hands - and some of the local government carparks are managed by private companies. But that doesn't change the
moral argument.
You will also notice that the council's policy refers to the registered keeper of the vehicle. The form for challenging a penalty notice states:
Please note that the registered keeper of the vehicle is liable for the charge, even if they were not the driver of the vehicle on the date of the contravention.
So I'm afraid smutchin, as the registered keeper, is definitely on the hook if it's a statutory penalty notice. He can try and claim back from his elderly mother-in-law if he wants. I don't know in the OP's case whether he's the registered keeper or his employer is - that will depend on the contract he's got. But if his employer is the registered keeper they have a moral duty to pay the charge - and they will no doubt claim it back from him.
If it is really the case that private companies have no legal right to levy excess parking charges then the whole basis of our legal system (which is based on the right of property owners to do, more or less, what they want with their land, as long as it doesn't infringe other people's rights too much) begins to be undermined. And our rush to privatise carparks needs to be stopped.