The problem has been found. (well, the main problem anyway)
The Woodruff key locating the flywheel had completely sheared.
Hence the timing was out on both cylinders.
It looks as though I probably did not do up the flywheel bolt tight enough?
Clearly the checking I did in "4)" above in the first post of this topic did not include pulling the flywheel off. Hmmm.

I cleaned off the faces of the Morse taper with Brakleen and put the flywheel back on with a new Woodruff key.
This time I used 3 dollops of lock-tite and did it up nice and tight.
I am used to flywheel bolts having a spring washer, but my setup at least, does not have a spring washer.

Anyway it fired up and ran quite good for about 10 seconds and then dropped onto one cylinder, then stopped.
I got it going with a push start down the hill, and it ran really well, but would keep dropping onto one or zero cylinders, especially at low revs.
I inadvertently pulled a few wheelies in first gear as both cylinders fired up all of a sudden at high-ish revs. Kind of fun.
I will look at it again on another day.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...