With all parts now fitted it was time to try her again.
No leaks, but still running flat out.
Eventually traced to the fact that some previous owner had screwed the governor adjustment screw on the carb right in (why do people do that ?). A few turns anti clockwise and it was soon ticking over nicely, albeit surging a bit as the governor was now working properly. I can live with that and I’m sure a bit of fine tuning will have it running sweetly.
The gearbox had a tablespoonful of oil and a similar amount of water. It now has the correct quantity of Castrol ST90.