Beware the Thistle car park and I speak from experience. Do not over stay even for a second, you will be charged £60 minimum and the appeals process is unlikely to succeed unless you have a cast iron case. You can't call them to discuss, the phone number they give is for automated payment only.
Time starts from when you enter the car park until you leave, all vehicles are photographed.
What happened to me is: I entered the car park, downloaded the app, paid on the app (this took 6 minutes), I checked the leaving time and left just before the leaving time. I was fined! I hadn't taken into account all the faffing around downloading the app. Thus I had over stayed based on total time in the car park, I should have deducted from the leaving time the time between entering the car park and confirmation of paying on the app. I would suggest an easy thing to do.