Just an update on this:
I sprayed some WD40 down the venting nozzles if the newer PCV on the car.
Initially it quieted down a little, but still had a regular (muted) 'honk'. Over the next couple of days it all disappeared though, so for now I'm happy
Note: on the diagram I posted above, number 10 hose plugs into the top of the valve cover at the rear of the left hand head. PCV function mystery continues....