Firstly, can you show us what was submitted as the appeal? If it revealed who was driving, no need to hide their identity any more on here.
Secondly, so that you're going into this with your eyes open - the IAS are often entirely useless, so are unlikely to take your side regardless of the strength of your appeal. If you're prepared to fight this, you need to be willing to go to court to defend your case, as PCM can be litigious (a loss there is usually around £200-220).
The sign isn't very clear that you need to have a session outside of these hours - if known, driver would have utilised the virtual visitor parking.
Those signs aren't great - a hodgepodge of all sorts of arcane terms that seemed designed to catch people out. The Consumer Rights Act (2015) says that:
(1)If a term in a consumer contract, or a consumer notice, could have different meanings, the meaning that is most favourable to the consumer is to prevail.
Although one thing that may count against you here is that the first term mentioned on the signage does seem fairly unequivocal that one must either display a permit, or register a valid parking session.
Considering some of the writing with the key terms on is rather small, the signs do seem quite far up. If you're wanting to pursue this avenue of challenge, then you should get some more of your own photos, showing how legible (or otherwise) they are when stood in front of them. I note your Imgur link includes a photo taken from inside the car, but that looks to have been taken from quite far away, and there's no requirement for the terms to be legible from inside the car.
If pursuing an argument based on signage it would also be wise to get some photos to demonstrate the layout of the signage at the site, to show whether or not it is prominent and 'there to be seen' - this is particularly important in cases like this where it might not be immediately apparent that you are entering private land.
