As far as I know it's the buyer's responsibility to ensure that the vehicle is legitimate before entering into an agreement to purchase it. Sounds to me like you are stuck with the HP repayments even though you don't have the car and probably won't get it again.
When entering into the agreement it was legitimate. It was after the agreement was signed that the car was stolen, and during the two days it was missing, used in a crime (OP said another as the first crime was the car itself being stolen)..... as far as I understand from the OP.
I'm not sure if this changes your rights any though (I'd doubt it)....
When my car was stolen, and recovered, the guards took a photo for me beside the car.
They said this was so that the car wouldn't have to be impounded until the people that stole it went to trial.
So they kept it about a day to do fingering printing etc, but i was able to collect it the next day.