If it was easy...
Tried rebooting. Problem recurs
The mouse is an optical one so no balls to fix.
Even tried removing the mouse altogether and relying on the touch pad (this is my laptop, since I'm on the road - again). Problem continues!
Doesn't seem to be the mouse, but something internal to the computer itself. Maybe the touchpad, maybe something more fundamental (i.e. expensive). I'll you know how it goes...