There is an argument sometimes made that the notice does not comply with the requirements of Schedule 4 of the Protection of Freedoms Act 2012, in order to hold the registered keeper liable for the charge, as it does not 'invite' the keeper to pay the charge, as required by 9(2)(e)(i) of the act. I'm not sure if this argument has been tested, and my personal view on it is that it would be a very brave person who sought to defend a charge solely on this ground.
I wonder if an argument could be made that as parking was forbidden during the times the vehicle was parked, no contract was formed, as no consideration was offered by ParkingEye (the sign isn't making a genuine offer to park for £100 when the store is closed).
The issue with both of the above suggestions is that neither ParkingEye nor POPLA would be likely to accept these arguments, so you would need to potentially be prepared to argue the matter in court.
Another thing to check, just to rule it out, is whether the land is within the boundary of the harbour, and thus potentially subject to byelaws. Given it's north of the motorway, I'd be surprised, but worth ruling out.