As I wrote in the previous post, the crank is tight in the cases and No1 appeared to be damaged in some way. So I removed the pistons and split the cases. When I split them the crank stayed in the upper case which is unusual.
The crank was tight in the upper case and wouldn't rotate. Further examination showed that rod No1 was bent.
I then had a closer look at the piston, It was broken and cracked internally.
When I split the cases I observed a lot of oil sitting in the bottom of the crankshaft area. This was unusual but I didn't think much of it at the time.
Looking at it now it seems that I had a case of 'Hydraulic Lock'. I don't know how the crankcase filled with oil. The only thing I can think of is that the oil tank contents had bled slowly through the oil pump and feed pipes and had slowly entered the crankcase over the many years of storage. Then when I've started the engine it has run briefly before enough oil was forced up through the transfer ports into the combustion chamber to fill the chamber with more oil than it could hold. As fluids can't compress the piston has travelled up and at a certain point could not travel further and has come to an extremely rapid stop. The inertia of the crank etc has then caused the connecting rod to bend and the little end to break the internals of the piston.
So now I'll have to get the crank rebuilt. I only need one conrod however they are generally sold in threes and are quite expensive. I'll shop around. I should be able to get a single piston and the bearing. Then I'll have the crank split the new rod installed and then the crank rebalanced. Hopefully then it will all go back together and run ok.