Free Traffic Legal Advice

Ever since the original 1991 Act came in, motorists have been plagued by PCNs issued for non-display, usually called "fluttering tickets", and they have proved to be very hard to get overturned, mainly due to the venality and rapacity of councils that despite have received the parking fee, still insist on the PCN being paid.

I've looked at off-street order for 2020, RQ008 - RQ010 and it has a clause in it that the parking ticket must be displayed at all times during the period the car is in the car park. There is a huge display of maps of the car parks, but I'm not sure which one you were in. However I think they will have it covered but you'll have to trawl through it to see if your car park is included.

There is Statutory Guidance on parking under the Traffic Management Act, (your PCN). YOu might care to look at the section on "Considering challenges, representations and appeals"
https://www.gov.uk/government/publications/civil-enforcement-of-parking-contraventions/guidance-for-local-authorities-on-enforcing-parking-restrictions#considering-challenges-representations-and-appeals
However, I don't think this would help you unless you can find out if Nottingham CC have a policy document on dealing with PCN challenges.

So, is a win likely if you stand your ground and take them all the way to adjudication ? My opinion is that it is not likely, but see what the others say. You're currently in for £25, and going all the way will cost you another £25 or nil if you win.