over charged for damages incorrectly

I hired a vehicle from europcar in December 2014 from the Newcastle Branch. I drove to durban. When in durban, 2 days after the car started giving me an engine problem and asked to change the car. They took extremely long to assist me with a new car. I ended up missing a show which i had purchased tickets for because of this. Then when I eventually get the new vehicle, they say that i had damaged the passenger lock and the lock was already damaged as i had noticed it everytime i had opened the door for my wife. It didn't look too bad so i assumed it was nothing. I then received an invoice for the lock on the car to which i had layed many complaints but were ignored. Money was taken from me for something which i had not done. I am complaining on this platform as i did not know something like this exists. I hope this complaint reaches them and i will never recommend anyone to se europcar as they are *******.