I said i did sign the document, but i had signed it at the service managers desk before id even seen the courtesy car, and was not aware it even contained any details about any existing damage as the service manager just mentioned it was for changing the insurance.
Invalid incorporation of an exclusion clause comes to mind. I haven't looked at this stuff for a while but the following case:
Curtis v Chemical Cleaning Co [1951] 1 KB 805
The plaintiff took a wedding dress to be cleaned by the defendants. She signed a piece of paper headed 'Receipt' after being told by the assistant that it exempted the cleaners from liability for damage to beads and sequins. The receipt in fact contained a clause excluding liability "for any damage howsoever arising". When the dress was returned it was badly stained. It was held that the cleaners could not escape liability for damage to the material of the dress by relying on the exemption clause because its scope had been misrepresented by the defendant's assistant.
If what you were signing was misrepresented it's not valid.
Though from the sound of that phonecall I think they've already acknowledged what they've done.