You are deciding whether to buy certainty for £25 or risk £50 to try to clear the PCN, and that matters because once the discount window goes you move into the formal enforcement route rather than a cheap exit.
The likely forum is the parking adjudicator, not court, unless this was not a council PCN. The burden means who has to persuade the adjudicator, and the standard means what is more likely than not. The real issue is simple: was the ticket properly displayed when the civil enforcement officer saw the car? If the sign required the ticket to remain readable, a photo showing it face down is the evidence they will rely on.
Your case is arguable, but not strong on the facts given. What helps you is that you had a valid ticket and say you found it face up on return. What hurts you is that the council has contemporaneous photos showing it face down, and a suspicion that the officer flipped it is still only suspicion unless you have something independent. The point about whether wardens can touch the vehicle is mostly noise unless you can prove mishandling.
I am assuming the car park terms allowed a ticket under a wiper to count as displayed. If the sign required it on the dashboard or inside the windscreen, your prospects drop to weak. If you have any independent, dated material - a passenger, CCTV, an immediate message, or photos taken before you disturbed anything - prospects improve.
Ask yourself three blunt questions. Are you prepared to risk £25 to fight the point? What independent, contemporaneous evidence do you actually have? And would you rather end it now for £25 than spend time and stress on principle?
My practical view is pay the £25 unless the sign clearly allowed a wiper display and you have evidence beyond your word. If you do fight, keep every photo, ask the council to preserve the CEO notes and all images, and if a Notice to Owner arrives make formal representations within 28 days. If that is rejected, you can appeal to the independent tribunal, where costs are normally not awarded unless someone behaves wholly unreasonably.