Yeah, I rented a car that got scratched while we were in a parking lot. Came out to find the scratch. Had "the works" rental insurance, and so thankfully was not charged for it. It's the little things in those contracts...oy!
Anyway, Europcar was going to get us 8 days at 197pounds and that included CDW and the liability waiver. That's pretty good, if I'm doing my conversion properly. If we were renting anywhere but in SoCal, I think I wouldn't be so paranoid. But stones get bounced up on the freeway and ding the windshield, punks in parking lots, etc....
How were you able to prove the damage was pre-existing? I assume it was noted on your rental sheet? And they still tried to charge you for that? Wow. Note to self - be sure to keep all the receipts!