That evidence would certainly have a good chance of winning on lack of prominent signage. Can you get your own contemporaneous photos of the signs, or lack of them?
Unfortunately not, as I am in the middle of moving house so don't have the capacity to travel back before tomorrow, which is when the 'discounted charge' expires.
Google images has a relatively recent photo of the car park, and I don't think much, if anything, has changed since.
Here's the view of the wall my car would have been facing as the driver entered and parked; the sign in the middle of the wall is just an Iceland one saying customer parking, while the Excel ones are are in the corners - one next to the generator, and another right at the edge, by the exit:
Also, as DWMB2 says, the entrance signage was off-set for a car who entered from the left side of the photo, as it was a one-way road - I don't think it's reasonable for the driver to look so far to the left like that as they were entering, especially with a passenger obscuring the vision.
Unfortunately I haven't heard anything back from the CEO.
Is there a template I could use to write the appeal tonight/tomorrow, so that I don't run out of time for the discounted rate? (the post didn't arrive until a few days ago, which is why this is so last minute). Also, by the time I receive a response, I will very likely be at a different address - is there a way to guarantee that they will send it to my next address, and not my current one?