Courtesy car rip off!! but is there a way i can get out of paying as it wasn't my fault!!!?

I recently recieved a courtesy car from a third party insurance company afer an accident that wasn't my fault. When they dropped the car off it was dark and the representative from the courtesy car company talked me through the damages on the car finishing with "its dark so i wont show you the damage as it will be too hard to see in this light" he then wrote this on his pda which i then signed. Three months later the car company pick up the courtesy car, unfortunately i was not availible to be there when they picked it up due to work commitments so they collected it from my mum. They picked up the keys and then checked the car over and this took half an hour! I then got an email a few days later saying that they had found some damage to the boot there was no fuel in it and that the car was dirty inside and out! I know i left it clean, polished, just under 1/4 tank of fuel and valeted interior. The bit that worries me is the damage but with it being dark do i have a get out clause?