I too have had problems with the 'o' ring at the the back of the head causing oil leaks. When I refitted my head, the gasket kit was missing the 'o' ring. Shortly after putting it all back together, there was a load of oil dripping down the back of the engine block. So, if you remove the head, strongly advise you replace this 'o' ring as a matter of course. It is not pleasant having to remove the head again just to replace a cheap 'o' ring!