4 months ago i swapped my saph cossie rep for a focus estate for boring family use

but this morning i receive a letter from a finance company stating there is hp outstanding on it

i dont know who the guy i got it off is as his name wasnt on the log book. it was the owner before him who owes the money.
and i got a cheeky hpi done at my local ford garage and it has a red mark against it so he will get a phone call soon about his hpi

what do i do?
and where do i stand legally as i dont wanna/cant afford to lose £1500 worth of car!