Problem Solved.
I had a dodgy spark plug - see pics.
Which for some reason worked when very cold - choke on.
As soon as auto choke finish car ran on 3 cylinders.
Then when warm it was fine.
Lesson learnt - plugs aren't expected to do north of 20k
Comparing Bosch W4DPO to the new NGK's
Dodgy plug you can see is really yellow - like it's burnt out...
Replacement NGJ BPR7EIX cost £40quish and work great!