Falsely accused of damage to car

Falsely accused of damage to car. The car I picked up was already extensively damaged on the wing and door on one side, which was listed on the paperwork as "dent 150mm+". As the damage was already listed, I saw no reason to report it more. On return of the car at Heathrow airport, they said they found damage that wasn't listed previously. Even though this "new damage" was just a few centimetres away from the rest, and had obviously all occured in the same, previous accident. Try to complain a few times, but they don't listen. Terrible service, and as a previoius customer of theirs for a long time, I will not be using them, or their partners again.