If you accept the cheque, you should wait six working days before assuming it is cleared. Only then should you release the vehicle.
A much better way is to ask the buyer to pay by CHAPS transfer. His bank will charge a fee, usually between �15 and �40, but the funds will clear the same day. Once your bank confirm receipt of the funds, you can let him drive away.
If they are reluctant to foot the bill, consider allowing them to reduce the payment by the amount of the bank charge. That way, it doesn't cost them anymore, and you have the peace of mind that the money is yours and can't bounce.