Supa 5 - new crank seals fitted depite that it still refuses to run for more than about 30 seconds, after which time it fouls the plug.
Carb has been checked and re checked, EVERYTHING checks out OK.
Having said that I did check the choke mechanism and think it might not have been fully closing - screwed the adjuster right in now, and probably going to give it a try in the next half an hour or so.
If it still fails I guess it is a case split to see if the cases are sealing properly and if the O ring between the cases is in good shape - I gather that if it is defective it can allow gearbox oil into the crankcase.
Might just put it to the back of the garage for a while..........