OK, so this what I have done this morning:
Using the pviseriii method, checked valve position at TDC - all good.
Reviewed my fitment of timing chain - exactly as stated in the Haynes manual. I wrote a note at the time, as I thought the description in the factory manual was unclear. However, my fitment exactly matches the photograph in the Haynes.
Made sure that there is minimal clearance on all vaves of 0.0015mm - it should be zero.
Checked the timing of actual spark. Impossible to stop manual engine rotation at precise point of spark, but a couple of nudges back brought the mark on the rotor to the scale in the chaincase. When moved to 31 degrees BTDC, the pick-up on the Pazon trigger plate is over one of the holes of the Pazon timing disc - wasted spark system, so impossible to get timing 180 degrees out.
Checked that the Keihin carb sliders were sliding. They do.
The fuel is fresh and the motor turns over freely and sweetly with the plugs removed. The resistance felt on the kick-start is as significant as may be expected with the plugs inserted.
Next step is to put the tank back on and have another go at starting the beast. If no joy, I am at a loss - push start?
Sooo....... As everything seems to be as it should (I have changed nothing), could it be something as stupid as my operation of the throttle? It is fairly warm here, so I have given just one twist on the accelerator pump and tried kicking it with no throttle, with just a touch and with a good handful, as I used on my old Commando with Amals. Plugs were cleaned and warmed between each session. I have no prior experience of Keihin carbs on a Commando.
As before, any input would be much appreciated.
Bob