If they took a car which they had been told did not belong to you (because, I assume, it is on HP) then they have acted illegally. You must tell the finance company which owns the car & they should be involved very rapidly in getting it returned.
When you get it back you should, if possible, keep it hidden (i.e. in a locked garage or somewhere away from your house where bailiffs are unlikely to find it). I know this may be very difficult or impossible but if you can do it it should help. However, if the bailiffs have it made clear to them that the car is not yours & they are made to return it then they should not try to take it again.