Just a quick update on this - I completely stripped the carb again just to make sure that everything is clean and the float level was OK (which it was) and rebuilt it with a new needle and damper and also filled it with SU SAE20 oil. When I started it up it ran much cleaner than before although there is still a little smoke which probably just needs a good run on the road to clear up. So it seems as if the damper has been the problem all the time
I'm not sure why this should be because I used nothing but 20/50 for the 11 years that I previously ran Minis and never had any bother - maybe the HIF carb has more damper rod clearance than the HS type because of it's ball bearings but other than that I have no idea. Anyway, it's hopefully OK now so fingers crossed... The next problem is getting the rear tub back on but the tail pipe of the GTM stainless exhaust I recently fitted is way out of line with the hole in the rear skirt 
I suppose the exhaust must be for the Mk2 and the hole in that is in a different position - never a dull minute!!