Short update...
Put compressed air through the crank-case ventilation hose, no blockage and I could hear air 'hissing' through the engine.
Took the oil filler cap off and with each squeeze of the air gun I could feel the air blowing up through the oil filler.
Don't know what the deal is, but the car runs well, blows no smoke, starts easily, idles smoothly, so I guess the blow-by through the rocker cover is just the way things are....! (134,000 kms on the engine)