Thank you Mudski and Bigcol for making the effort to reply.
Bigcol, I take it you mean the fuel pump in the injector pump assy when you talk of lift pump. I thought the fuel draing back to the tank would only go from the filter back to the tank. You are suggesting it may leak back via the injector pump, is that right?
My Patrol has done some 245000km, so I think the injectors are a possible cause of the smoke problem. Did some more checking today and found that with the Patrol stationary, and reving the engine by hand at the injector pump assy to around 1200rpm, I can detect an occasional miss, reving to 2000 and above the engine runs fine. Suspect injectors again, would you agree? Checked the glow plugs only electrically. Have not removed them, in fact did not know they could be cleaned. I'm not too savvy with the Diesels yet, always keen to learn.
Thanks so much to all of you who have replied. 14/7/2015