Geoff,
It is also possible that the ball, normally under pressure by the spring
inside the selector ball unit, has dropped down...., and is happily dancing
around on top of the selector plate or-shafts. Or it falls to the bottom
part of the gearbox housing, hopefully not damaging anything in between.
This happenend some years ago with my 403 BWCR5 gearbox. I only could drive
home in first gear, because the loss ball blocked further gearchange.
So if the problem occurred suddenly, I recommend to stop driving, to
dismantle the gearbox topcover ( only 8 nuts) , and to have a look first.
New selector baal units (there are three of them in topcover) do not come
cheap, but some years ago I could order new ones from Brian Marelli at
Bristol Car Services.
I renewed all three, the new springs have better tension, and gears do not
drop out any more.

Jaap in the Netherlands.