Hi Peter,
You say the gear lever has come off in your hand. If it had you would have been able to see that it is held on to the gearbox by an annular screw cap (sorry if that sound a bit rude).
Is it that just the top part of the gear lever has detached from the lower section like mine? There is a sleeved joint with a layer of rubber - I suppose an anti-vibration measure - which can separate. (Shown hatched on the extract from the Haynes attached).
I got a replacement from Fred but I think it could be rejoined using glue - the Gorilla stuff seems to work OK in flexible areas
Mike HC 78