What is the classic car hire security deposit for?

The security deposit is to cover the insurance excess. Details are on the booking form. This is refundable on return of the car without damage. If the hirer cannot refuel the car, receives any parking or speeding fines, or causes any minor damage to the car, then we need to be able to recover these costs. It also allows us to charge any excess on insurance claims, should the hirer refuse to pay any invoice raised against damage done up to the excess limit.