To the OP, here's another thought.
If you bought the car as a private individual (i.e. not as a business), and if you took out finance, or even if you only paid the deposit by credit card, then the Consumer Credit Act will apply and your finance provider or your card issuer will be liable for the transaction - if this is the case, then report the incident to them and they will fight your case for you.
However, if you paid both the deposit and the payment for the car via bank transfer or cash (or debit card), then the above will not apply to you.