| Sorted!
Neil at Cornerspeed (01623 758877) suggested cleaning the connector contacts for the crank position sensor or the throttle sensor- it turned out to be the latter, green corrosion cleaned off & WD40'd and it's like a new bike. I'll have to give it a run (Wales next w/end for a stag do) to be sure, but it's actually rideable now, so I'm confident it's fixed.
Thanks to Neil and everybody else who contributed...
You SHALL go to the ball, Cinderella! | 
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