You can obviously appeal but bear in mind the answer below. The reality is that if you parked there without displaying a permit then you do not have a positive defence.
As to the identity of the driver. This used to be the way in which private land fine claims fell. They had to identify the driver. Unfortunately the protection of freedoms act came into force in 2012 which creates a presumption that the registered keeper is the liable person if he does not name the driver within a specified period of time . To be honest there were cases even before that the did a similar thing
This is a private land fine. There is a good of nonsense on the internet about private land fines. The truth is that they always did have a perfectly valid claim in breach of contract against those who do not pay or park at variance with their terms and conditions. The issue was that it was not very cost effective for them to sue. It still is not.
They used to sue almost nobody but they are becoming more aggressive now largely because they have lost control of the industry and can only regain it by suing those who don't pay. The chances that they will sue remain very low though so the risk is not great.
They have been emboldened by the case of Beavis which you can view here
Although all that really does is confirm that £85 is not excessive.
If they do sue, you can plead your case and may well win. If you don't win then you will only be liable for the sum of the fine plus about £25 in court costs and £55 in fixed solicitors costs. If you pay it off within 28 days of judgment then it will not show on your credit file.
They do have an appeal option called POPLA. I have never heard of much success from them although apparently they do uphold many appeals. Their decision does not determine whether you will pay anyway. The order of the court does.
You do have the option of putting them to proof on the adequacy of signs though. That may well succeed.
Alternatively, you can offer them £10 in full and final settlement. You need to make this offer in writing heading “without prejudice”. Mark the cheque “full and final settlement” and take a photograph of both sides of it. If they cash it then there is a pretty overwhelming argument that settles the claim.
You could appeal to POPLA to use their discretionary grounds to cancel though.
Can I clarify anything for you?
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.