It was the ignition live wire going into the plug. Totally corroded. When I took the side faring off, it fell apart. Seems previous owner had been having difficulties with that particular plug and made a bit of a hash sorting it out. All fine now though.
Rode it to work and back yesterday and apart from the idle needing turning down and the front brake lever needing adjusting everything was sublime.
(Apart from my dodgy gear changes due to a swollen/bruised/broken left hand.)