...And finally I won
There was a new pattern one inside, with 3.8mm wire, and it resisted all attempts to remove it without causing bodily harm. Until I used a section of thin steel tubing that went over the hook end of the spring, moved it clockwise and so freed the kick starter spindle.
A NOS one made of 1/8" wire was then installed with much less pain.
By the way: there is a small inbus screw inserted, where the pushrod goes out, see pic; what is the purpose of it?