The issue:
I turned the key, the motor started cranking. After a few turns I checked the carb; while opening the throttle lever gas squished out - seems to be working. Now I wanted to properly start the engine and cranked it again. It immediately fired up but died as quickly again. I did this a few more times, always with the same result. It seems to come alive but dies immediately. Apart from the vacuum advance hose (which has to be replaced as it is too short and brittle anyway), valve cover breather and the transmission linkage I think everything is hooked up.
Timo