Originally Posted by ollyit
I have a very slight left and right movement a tiny gap probably 1mm at the front section near the prop end
Unfortunately that does sound like a classic splines stuck in the crank situation. - Sorry.
You don't really have many options apart from warming the engine up and cooling it down a few times. As the engine hasn't been run in a few years this may release it if its not too bad. It could help to get the engine in a wheely bin as deep as possible to flood the exhaust chamber with water to cool the shaft.
Some absolute last resorts:-
-Source another bottom end and destroy the old one getting it off to gain better access to the drive shaft.
-Strip the powerhead in situ, splitting the crankcases from around the crank and hope you can get it off on its own.
-Turn the engine upside-down and fill the exhaust with diesel, some of which may find its way to the join and loosen it off. Although you'll also fill the cooling system and crankcase with diesel via the exhaust ports.
-Ebay the engine.