You seem to be doing the right things John. In these circumstances I would suggest holding the flywheel as rigidly as possible - it's surprisingly easy to move even a chunky friend with a bit of leverage. Can you bolt the engine back in the frame, for example? And do use a spanner that fits the nut well, once it's rounded off there won't be much you can do. Has someone used Loctite on it I wonder? Either way heat is your friend.
(Sorry - you seem to have fixed it while I was typing - well done!)
(Sorry - you seem to have fixed it while I was typing - well done!)