Johnson 20 Non starter :-(

Hi folks, I have recently acquired a Johnson 20 of unknown age (somebody reckons it's an '83). It was running pefectly but not charging. I had the motor running sweetly in a bucket, stopped it, went to restart and nothing. I have checked both plugs, they spark with a bright blue spark, petrol is getting in to the bores, I can see the fuel exit the plug holes when I crank it with the plugs out. I know the fuel is good. To my mind the only thing left is ignition timing. I hooked up my strobe to the top cylinder, it seems like the timing marks line up with the triangular piece cast in to the pull start assembly - Is this correct?
I have cleaned the carb and checked everything a dozen times but it will not even try to fire. I think the flywheel has slipped but don't have a puller to check it
Anybody got any suggestions? Can't find anything in the manual that I have not already tried.
Thanks in advance,
Dave in the UK.