I had a similar thing in Bristol a couple of months ago. I used the car park by the Thekla when going to the Hole in the Wall for my wife's leaving work drinks. Anyway, paid on the app but ended up being there longer than intended. The app sent me an imminent expiry warning message, but somehow in going back in to pay, I managed to pay for the car park by the Portwall Tavern. I was fuming when I got back to my car and saw the ticket, but realised my error in the morning.
I wrote to the council, using some screenshots from the app and said, if the wanted to fine me I understood, but I wanted the payment back for the car park I didn't use. They came back after a month or so and waived the fine, as they could see that I had tried to pay in good faith.
I guess it's down to the individual council.