On going to our local hospital last night the pay machine was "out of order" knowing these people are very quick to send out failure to pay tickets, the other car park machine was the other side of the hospital, I took a photo of my car that was near the pay machine, then took a photo of the " Out of Order" on the machine + the machine Number, at the other car park bought a ticket & took a photo of the details for my own evidence, don't know if this dull bit of info will help you.
If you didn't go back and put the ticket on the windscreen I can't see any reason why they shouldn't send you a "fine". You can then go to all the bother of challenging the charge.
Of course, they could say you photographed someone else's ticket (unless you have to put in your registration number.
I think "pay-and-display" car parks should be made illegal. They make you put a sign in your windscreen which says "I won't be back for 2 hours, it's safe to break into my car". On top of that you have to guess how long you'll be away - not easy for hospital appointments which rarely seem to be on time and also difficult if you go to visit a town you've never visited before or a tourist attraction.