I was thinking about the gear box problem. It was strange how the gear lever had no spring back. So I removed the right hand crankcase cover, and the clutch. There was a spring behind the selector arm but it didn't look right and didn't seem to be doing much but I couldn't remember how it should look. I went to the workshop manual, no use. It had an exploded view of the components but not how they actually fitted together.
Now I took quite a lot of photos of the engine during disassembly and went through them. I came to the one below.
You can see the selector arm and that the spring is tight on either side of a fixed pivot on the crankcase. Mine wasn't like that it was flopping around. I must not have assembled it correctly. So I placed it in the correct position and now the lever springs back to where it should be after changing gear. I'll put it back together today and see if it works.