OK. It starts. It won't run without the choke on, but we are making progress.
Astonishingly it appeared to have diesel in it. I am amazed by this. I have never had diesel in a can, and that bike has been fuelled from cans only. So either there was some diesel or contaminated fuel in it when it came home after being lent out, or I filled a can with diesel without noticing, which seems unlikely. Another possibility is that someone put two stroke in it. Anyway, with new fuel in it, at least it will start.
Watch this space for the next exciting update.