Hi Dave (or anyone else with ideas)
I can move the gear lever to all three positions (four including neutral). I can see the gears sliding, through the cap.
With the clutch lifted I can rotate the back wheel without the primary chain moving.
With clutch engaged when I rotate the back wheel the primary chain moves in all four positions.
Seems I have lost neutral.
Cannot understand how that happens so guess I must remove gearbox and have a look inside.