I am pretty sure that IF the buyer is unaware that there is finance owed on the car then the finance company connot take the car, it is their responsibility to chase the person who took out the finance.

Keep hold of the car and if they turn up, do not let them take it.

In the meantime, speak to citizens advice and find out your rights, once you have a full understanding of your rights it will mean you are less of a push over for the finance company.

A mechanic mate of mine had similar but it was a customers car, he got a call of the lad to say the car is going to be repo'd and to let it go.

When they arrived to take it he refused to let it go saying their was a bill for works done to the car and if the finance company owned it, they needed to pay it. He got a £3k car for £500 in the end.

Personally I would get some concrete facts on the case in hand and take it from there.