Hi Robo,
I run a YB Turbo in a Dax Sprint and Hillclimb car. We had the same problem some time ago. We ended up fitting a cometic gasket plus the half moons. We also used a very good gasket seal product called Wellseal. Its not cheap (£20) ish but is very good. We put the wellseal on the underside of the half moons and the cometic gasket with a bit extra around the area where the half moons join the gasket.
My engine is around 300bhp and we have struggled to keep the oil in it ! and are currently reviewing the breather setup.
The place to look closely is where the half moons join the cometic just behind the cam pulleys.