It looks like I have solved the problem...
On closer inspection I noticed that the air box at the back of the carb was not fitting properly on the right hand side of the carb intake.. it was difficult to see but the carb was not snug inside the plastic/rubber air box and may have been leaking air.
It was very difficult to remedy and I had to heat the air box and force it into position by releasing the front intake rubber and moving the carb rearwards, tightening the large hose clamp, then moving the carb forward onto the rubber intake manifold and retightening that..
The air box appears to be deformed and I shall have to remove the seat and tank tomorrow and try heating it up again to get it into a better shape to fit its intended purpose.
The good news is it has solved the high idle with choke issue!
Different carb but useful info..
https://www.youtube.com/watch?v=Uid8L4yMvfc
PS. The next day it was 12 degrees warmer... up from minus 2.
This had a significant affect on choke idle speed which was reduced.. Maybe air density plays a part?
Anyway all good now...