numpad doesn't work randomly under VNC

Bug #192508 in X.Org X server: “mouse keys turns on randomly”

In keyboard preferences > layouts > kb layout options > caps lock key
behaviour
I had caps lock behavior set to "caps lock toggles Shift so all keys are
affected".
This is not the default setting (although I can't remember changing it).
As a result pressing the numlock key while caps lock was enabled in effect
triggers a shift+numlock which enables mousekeys.
Setting the caps lock key behaviour back to default ensures that numlock
only enables/disables numlock even with capslock enabled (as you'd expect).
This almost certainly solved it for me.