No signs ect.
Any photos to show this? If you're going to try to challenge the matter on the basis of no (or insufficient) signage, then you'll need to be able to evidence this.
From a quick look at Google Street View the road in question does appear to have some signage
https://maps.app.goo.gl/1hFKAKvVTW7QrKE67, but one sign on its own could certainly be argued to be insufficient, if you can show there were no others, or they were all poorly placed/not readily visible.
'No Unauthorised Parking' is also interesting - depending on what the other signs say (if present), there could be an argument here about the signage being forbidding, and therefore not being capable of creating a contract.
So that you are going into this with your eyes open, neither PCM, nor the IAS are likely to uphold your appeals, so if you want to fight this all the way you'll need to be prepared to go to court.