Ralph H. gives this update:

So here is an interesting turn of events that led to the mower going t..s up.
Despite a full tear-down and many repairs over the winter the float in the carb must have stuck open. this led to a flooded air intake that was then sucked into the cylinder on first quarter turn that then locked the starter from turning any further as it could not compress the liquid. I think it is called hydro-lock???? The penny dropped when I saw the carb dripping gas from the overflowing bowl so I pulled the dripping wet plug cranked it over and blew out the gas reinstalled the spark plug and it fired on first turn of the key.