The feed tube has a Peter channel on it already and it helps a bit - not on the original photo. As can be seen the manifold is much bigger than on most builds which generally constrict considerably at the barrel support, where as the manifold in mine is open all around the base of the barrel. offering minimal resistance and plenty of thermal mass to dump its heat and sink for exit to the horizontal flue.
The internal dimension of the heat riser is 6" and the flue out is 6" - its an optical illusion that suggests they are different as the feed out hole is low down. The heat riser is made of pumice and is around 2.5" inches across so it meets the spec for insulation. The internal surface of the heat riser is white after burning, and it is only the outer surface which has any blacking. From my reading few people get away without a bit of soot in the cool areas so I am not really concerned about inadequate burn temperatures. The pumice in the J-tube heats up quicker and stays hotter than firebricks and the sacrificial stainless flue liner in the burn tunnel glows red hot after about 20mins.
I decided to take your advice regarding trying a flue extension to the outside section - so I added another six foot. The outside flue is now 12'. I did another test burn and had a three hour burn with no significant smoke back. I had it burning with a lid on the feed tube for most of the time - and without it there was still a bit of smoking - with it on it burnt for extended periods without smoking. This is significant as previously with a lid on after a short time the feed pot got so hot that it smoked and the reverse flue action took over. I assume that two things have changed - the flue extension has tipped it over to been adequate draw and the thermal mass is behaving better as it has been burnt for two days running. Previously I was getting steam out of the top of the external flue, now there is no visible steam which means it is condensing before it gets to the flue exit. Effectively its a 100% clean burn at this stage.
So thanks for the perseverance and I can now proceed to apply the final plaster coat and get a proper lid for the feed pot.