Unfortunately 5lab, you are wrong. The law was changed recently.
The dealer would, in your case, have to proove beyond all reasonable doubt that the car did not have a damaged head gasket when it was sold. This is so difficult to do, that effectively, he must accept responsbility for it. This applies up to 6 months after purchase, when it then becomes the customers responsbility to proove it was there at the time of purchase.
It sucks, and its why lots of garages no longer sell sub £1000 cars. It isn't worth the hassle.