Your contract with the garage was that you would pay them a certain amount of money in return for receiving a car with specific specifications. By not providing such a car, they've broken the contract and you're entitled to a full refund of your money. The fact the fault may be due to an error by the manufacturer is irrelevant. The garage must refund your money. It's then up to them to seek compensation from the manufacturer but that has nothing to do with you; the garage must refund your money even if the manufacturer goes bust and they can't get a penny back themselves.
Contact your nearest Trading Standards office if you continue to experience problems:
http://www.tradingstandards.gov.uk/search/dbas e/searchlocal.cfm
(It might also help if you drop large hints about involving the local press. The threat of bad publicity might move things on faster than Trading Standards can do).
Chris