Are you in Scotland or England?
If Scotland, ignore it. Do not respond to it. Do not pay it. They will not take you to court over it as the laws over parking are very different and even if they did, consumer law dictates that the owner of the car park can only seek financial redress equivalent to what a paying customer would have paid and even then, only if the car park was full at the time you used it, which I'm betting they aren't.
£100 for an overstay that in real terms costs £1 an hour? That'd be ****ing right!
England...? God knows.