The Serious Fraud Office is ---> thataway! No, seriously -- if they've taken a deposit from you, and given you a receipt with the details of sale, the deposit amount and total sale price indicated, they're bound by contract. If they then sell the car to someone else, you've got 'em over a barrel.
'Course, if they've gone bust, you're stuffed anyway. If they've just closed, well, you've got options. Nasty ones. From their PoV, that is.
I'd not consider doing the BACS/CHAPS thing with any firm I didn't trust to stay in business and not to screw me over.
Jon