Well, it's never done that before...

[Fazerider]

It was all pointing towards a duff CDI, though I too had doubts about that since it's encapsulated and the amount of vibration required to cause it to fail would have shaken the entire bike to bits long before that.
The only wire I'd not checked going to the unit was the thinner black one. According to the diagram this goes to the ignition switch, light switch and the optional alarm. I'd already checked the link plug at the back end of the harness was OK, but then twigged that if it needs a link plug if the alarm isn't fitted, it must also be an immobiliser... so that black wire must be able to inhibit the sparks. Lifting up the harness I found a second link plug underneath with one of the two black wires going to it completely severed where it had been chafing against the edge of the plastic tray where the CDI unit lives.





Chopped the whole connector off, soldered the two wires together and then insulated it with heatshrink and silicone. Pressed the starter button and, once my hearing had recovered from the immense bang as the vapour accumulated in the exhaust detonated, I could hear the engine running. :woot


So, thanks all for your suggestions. It's a great relief to be back on two wheels.