Violence wont solve anything in this situation, it will actually turn the law against you instead of him.
Ring Trading Standards, they are the defenders of us, the punters. Seriously they're like terriers once they get wind of a dodgy dealer!
If this doesn't work, threaten legal.. Remember if he is truely in the wrong then most of your legal expenses will fall to him!
If all else fails.. You'll have to destroy the cars. As someone else said if you can't get it back he wont be getting it either.
Also, link to this page from every classic car enthusiast's forum.. Especially all the Scottish ones! Not just Ford ones either!