Success! I have it running and it's starting easily and running very well.
The problem was the ignition timing. I had it about 3 degree out, as I was using the point at which the magnet passed under the edge of the hall sensor sleeve/bracket. I moved it to the point where the magnet was under the centre of the hall sensor.
It started almost immediately.
Now it's looking like I can choke/prime with the ignition off and the decompression valve depressed. A certain number of times that will be found after some practice. Then, pull the decomp valve out, turn the ignition on, and flick with a chicken stick. Two flicks and it goes. Very happy about this, I've been walking around laughing about how good it is to get it going.
The engine sounds like it is loading up suitably with the 26 x 10 prop, as far as can be assessed on the bench. I was only game to give it about 1/4 throttle, as it has so much power and my bench mounting set up is a bit primitive.
Vimeo video here:

https://vimeo.com/167048008