As previously indicated, ignition timing problem very unlikely due to healthy sparks being produced when testing each plug on its lead.
Furthermore the battery cranks the engine robustly, so it also is in excellent nick.
My semi-ignorance of things electrical hasn't prevented me from considering the regulator, but reading elsewhere online tells me that even if the regulator was on the blink / failed, the battery alone would still power the ignition system to produce sparks, provided that the voltage was above 9v.
Thanks for the idea, though.