Had another look at the pressure transfer plug under the clutch. It has a very fine gauze filter on the crankcase side. In trying to remove it intact I destroyed it. Blew it all out again with compressed air and seemed to gain some extra airflow. Reassembled and gave it a run, and whilst the oil did flow, it was still insufficient. Best of all however is that I now get a "psst" when I crack the oil filler cap.
Will keep at it and post results.
Jim
