You will need to check the contract - but usually once half of the total amount payable (cash price of the car plus interest and charges) has been paid off - then they have a right to inform the finance company in writing and to return the car. The finance company will have to take the car back - although they will charge for any damage that they decide is not fair wear and tear (they can be very strict on this too). This is part of your rights under the comsumer credit act 1974.
I would say that if they are 42 months into a 60 month agreement that there could be equity in the car - so it may be worthwhile finding out about selling the car and settling off the debt that way. You will need to get somebody to value the car and decide if it is worth doing