Check the date of the GCV photo. Things could have changed since the photo was taken.
If it is private land then you should seek out any signs that contain the terms and conditions for parking on the private land. Which PPC operates there? You would need to check the CoP of the relevant ATA that the PPC belongs to.
You state that the car park is free but you also know that it has a 2-hour limit. If you know that then you must know which PPC.
Anyone parking on private land should have the common sense to check for terms and conditions. Whilst the dearth of signage is a useful appeal and defence point should it come to that, it is always best to avoid getting to that stage in the first place.