It sounds like you have a good case to get the deposit back. The easiest way would be if the garage also agreed.
If you ordered the car over the phone, you could mention the distance selling regulations. You could also argue that, if they are not going to lose on the transaction, they cannot justify retaining the deposit (I'm sure that they will then deny saying they would make double the amount).
If they wont agree, you should write stating your case (including mention of their inconsistencies) and stating that you would like to complete the transaction as originally agreed and, if it not possible to come close to that, you will not be taking delivery of the vehicle and you require a full refund of the deposit. If you paid the deposit by credit cards, you could then dispute the transaction through your credit card company. You can also contact ConsumerDirect and, as a last resort, you could give the garage 30 days to refund the deposit otherwise you will use the courts.
http://www.moneyclaim.gov.uk/