Unreasonable Charge, Cheating

On the 17th of august, 2014, I went to collect the car I had hired from this company: The Sheraton Heathrow hotel branch. After driving for approximately 30 miles, the car began to release smoke from the front bumper. I called for the company's car rescue service and the car was towed back to the Sheraton Heathrow Hotel branch. The tow person informed me that it was likely clutch damage, and that this damage should not have been caused by such a short driving distance. This means that the car I had been supplied with by the company must have been faulty to start with, something I am not happy with as it caused us a great deal of stress as well as potential danger. However, once I arrived at the branch, the reception claimed that I had caused the damage to the clutch and that I had to pay £500, straight away. I was very unhappy with this accusation, and with the fact that the receptionist refused to replace the car unless I paid this money right away - furthermore, I am an experienced driver with 20 years of driving under my belt, and I'm sure that I know how to work a clutch.
Lastly, I was dissatisfied due to the fact that I had paid for full insurance, and when buying it I was reassured that it covered absolutely everything-yet when the issue arose, your company claimed clutch damage was not listed under the terms. No paper policies or terms related to this full car insurance were presented to me to back up this claim. However, out of necessity, reluctance to waste even more of my own holiday time over the issue, and the fact that I needed a car to travel, I paid the £500.
I do not agree to accept the liability of the damaged clutch, without a proper inspection by qualified engineer.