Phil - Thanks for the response. I don't think its the blowby problem. There is a tube situated to the rear of my back
pot joined to a nipple right on the inner engine casing.
This is where the blowback is originating from. I had the compression tested recently as leakage here would have been one explanation for excessive pressure at the crank. But this was fine.

Does anyone know what would be the pros/cons of routing this pipe back to the oil tank and providing a breather at the tank end?

Chris