Deceptive practices

I recently rented a car from Gatwick in the UK.I paid for full insurance. On returning the car they pointed out "scuff marks" which could have been removed by a magic eraser. Despite the fact that I had full insurance they sent me threatening emails suggesting that I would be liable for damages. I will never, ever use Europcar again. this "scuff" routine seems to be the standard way they operate. Buyer beware!!!!