ok... im no expert, but here goes of my understanding... (other has done business law)
a shop can put a 99p price tag on something, when you take it to the till they can say its £99, you make an offer and either agree to pay it or walk away.
he accepts it.
on the receipt does it have an outstanding amount on it...
ie when paying for a holiday is states £600 holiday price, £100 deposit, £500 balance to pay.. etc.
if your paperwork completed by them has an agreed price i would think you/he have entered in to a contract at that price and the deposit is a sign of good will. However i am guessing that until the total price has been paid he can withdraw from the sale (as per houses).
Citizens advice i think.
just had a quick check, is the additional monies to pay for work to make the car pass its MOT, in which case you should be able to take it sold as seen or pay for the work required.
Trading standards at your local council would be worth a quick contact... you may well not be the first.