Thanks for the tip, how quickly I forget my basic mechanic skills.
Tried with Start Ya Bastard & got it to fire first try, ran for about 10 seconds.
Tried a second time , and noticed it wasn't revving up to what I would expect at full throttle (no, not up to choke position). Whilst it was running, manually moved the governor bar & managed to achieve what I would expect to be normal revs, as well as running for about 30 secs before I stopped it.
Am I looking at something in the governor linkages, or something I missed with the carby ???