Giving this a bump as I have just won an appeal, through POPLA, against Excel Parking at the Peel Centre in Stockport.
Parked there last December and had no idea it was Pay and Display. It was busy (Christmas shopping,) dark and I made an honest mistake- just didn't realise you had to pay and didn't see any signs.
Excel threatened me with numerous letters, wanting £100 for a £2 stay.
I challenged them and they rejected my appeal so I requested a POPLA code/appeal which I subsequently won. Result: Charge cancelled. This decision is binding.
For anyone else appealing against this company the argument to make is that the parking charge is not a genuine pre-estimate of loss. Many people are beating them on this fact. Plenty of info on the forums mentioned above.
You can beat the man.
Hope this helps anyone else in the same situation.