Beware of hidden charges - They have your huge deposit!

I hired at 3 berth Motor home from Nationalmotorhomes from the Heathrow depot in Walton on Thames. There was a lot of paper work to clear before hire but eventually, come the hire start day all was ready. A very brief tour of the vehicle was given, most of which, if one hasn't had the experience of hiring one of these vehicles before, when straight over my head, Although most of the instructions were common sense and all the necessary manuals and documentation were pointed out to me lying in a draw, there was no major difficulties .
Holiday went well.
However on my return a 2" scratch was noted on the vehicle part called a "Luton Leg". this scratch was at the level of the bottom of the passenger door. The staff at the deport ,upon returning, suggested that this was not a tear done by driving, They suggested that in one of the 6 or so caravanning camps that I visited it would probably have been caused by children playing. I certainly did not have any idea this was down to driving nor was I aware of any incident that could have made this scratch whist I was at the wheel.
After several emails to and fro the motorhome centre I have been charged with £810 incl VAT for the replacement part.
I requested an invoice for the new part but of course I was presented with a made up, undated, invoice.
At all stages during our corresponding I was told that I had "agreed to the hire contract" and therefor that was that and all this I agreed to. But £810 for a new part?
I queried their statement regarding fair wear and tear but got no sympathy. I requested a common sense approach to suggest where does "fair ware and tear" play its role and wilful damage begins. I got nowhere.

Please do not hire with this company.

The picking up of the vehicle and bringing it back is all fairly laid back and friendly but remember they have your huge deposit from which they will filter off as much as they can get away with.
The basic hire of the vehicle cost £750 for the week, on top of that cost I now have to pay £810 for a new part that I have no idea when and how it accumulated its 2" scratch.