Clare's idea about selling the vehicle through a mate is a good one, but maybe doesn't give you entire protection.

Legally if you buy a car privately and the seller owes HP on it so doesn't own it, the HP company is perfectly entitled to take their car back whether you bought it in good faith or not. You then have to try to get your money back from the seller.

If you buy the car from a trader, though, the law deems it that they should have satisfied themselves to good title before selling it on. If that hasn't been done but you bought in good faith, you can keep the car and the HP company can recover their money from the trader.

I'm not sure that putting an extra private sale in will help, and a trader mate probably won't want to end up outt of pocket.