Hi Nigel
rocker breathers are fitted because the rider blames oil leaks up there on excess pressure, divided opinion on this, myself I think the blame lies elsewhere but who knows I don't flog the motor as much as our racer friends may do so could be a difference of usage applies.
I had your problem for some time on my latest project, difficult to see where the leak was coming from, has to be said the surfaces on the head were less than ideal.
I cured it by fitting two layers of gasket, applied Loctite 5910 to both the head and the rocker box and stuck gaskets to both, left in between the gasket dry (rocker box will need to be removed to re-torque the head).
This worked, I have since had to remove the head for another reason and the gasket parted just fine