Hi
If you are saying you signed a contract for a specific car, paid a deposit and agreed to proceed with the purchase subject to inspection, the audi garage have broken that contract. So the contract should be cancelled and the £500 refunded. The garage if they want to maintain your business, should be looking to find you an alternative. Once they find one, you would need to start the process again.
Very frustrating, but I think perhaps you could use this to your advantage when negotiating the price of the new car when they find one.
Huckster