After much deliberation, I think I have identified the sequence of events that leads to the disappearing mouse cursor.
If I leave the computer without logging out, eventually it will sleep and require a password to get back in to my desktop.
However, in the meantime, my son wakes the computer and sees it requesting my password.
However, he switches user to himself, logs in, does some work and eventually logs out.
I come back and find I am still logged in, but have to enter my password. I do so and get my desktop, but no mouse cursor. I can move the mouse up and it does activate the menu (though the mouse cursor is still not available). I manage to logout and then log back in, at which point everything is fine again.
A strange effect from a set of circumstances that has been common for the last couple of years. I don't recall this happening on 14.04, 14.10, 15.04 or 15.10.
