thanks for the help and persistence;
what I would say is that it is defo not the motor; the motor spins every time I short out the relay output to battery+. Every time; I've lost count, maybe over 100 times.
I have looked at the wiring diagram on 8-11 in the Yam shop manual. Ive tested, as much as I can, the terminals and looms. With ignition on, run switch on, I "hear" the cut off relay energise and I confirm voltage and continuity.
When I press the start button I hear the clunk of the starter relay , and confirm voltage and continuity. So no issues with any of the switches and cut-off relay otherwise the starter relay wouldnt energise when I press start.
When I get no engine start, I hold the start button (no spin) then whilst holding I short the relay+ and battery+ with a pair of big pliers, motor spins - this works every time the start switch fails to spin the motor; It cant be the motor or the cable to it from the starter relay. I cant be anything "upstream" from the starter relay or the relay would fail to energise when I press start (according the the wiring diagram)
All I can think of is some intermittent mechanical failure inside the starter relay moving parts. I just dont get it. Its ridiculously illogical. I think I'll have to replace the starter relay anyway . So I have ordered a second hand one from eBay for £5 with 28day warranty. If it works, and removes the problem for 28days, I will buy a new one.