Charging me for damage I know nothing about

Make sure that you dont use ANY national car rental, Cars are good but you have to watch every move they make! As for me i got told not to bother filling up on returning the vehicle as they can do it cheaper by 5p per litre. It was a no brainer as i thought. BUT what they dont mention ( until now) is that you will be charged for a FULL TANK no matter how much is in when you hand over. in my case half a tank full. Its a nice little earner for them. Me ? wont be getting any more business from yours truly. Just a con!!!