Had to happen one day

I have traveled abroad both for business and pleasure often hiring cars. I have just come back from Poland and a few days later I checked my credit card statment and the car hire company had taken £300 of the card.

On collection of the car I was told it was brand new and had no damage but checking itover I found various damage which I reported.

On dropping the car off the office was shut and a note said to post the keys.

I contacted the broker who sent me some photos of faint scratches on the skirt under the bumper.

I had not noticed these on collection or during the hire,

While I have insurance to cover me I wonder how I stand legally any one with experience of being charged.

I got charged by Sixt once when the string holding up the percel shelf snapped but they refunded me. Someone stole the areial of my hire car in Finland but Avis did not charge me and I caused very minor damage to the wheel cover in POrtugal when i clipped a curb but again the hire company said they did not charge for minor damage. So this is a first!

Advice?