Pushing the Start button lights up the dash normally, but there is no engine cranking.
Attempted to diagnose the problem to be either north or south of the starter relay. The relay (bottom right of the relay box in the engine bay) was removed and tested - contacts and coil all good. Relay reinserted, and added two small wires across the coil contacts, coupled to an LED, to indicate coil power. On the next attempt to start, the LED lit and the engine started normally.
I can only conclude the relay is slightly sticky. No signs of corrosion anywhere electrical. I guess it might also be the clutch switch, but there isn't a fault to find at the moment.