[ubuntu] I can't swap my left and right mouse buttons?

I am using 16.04 and have a Polaroid wireless mouse, which has worked perfectly until I rebooted my laptop about fifteen minutes ago. When it started back up, my mouse buttons were the wrong way around (as a leftie, the first thing I do when I install a new system is to change the buttons so that the right side is the primary button).

I've tried going into Mouse settings, and no matter whether I click 'left' or 'right', the mouse stays the same. I've tried creating an 'Xmodmap' file in my Home folder and putting in a 'pointer 3 2 1' line, and it stays the same.

Can anyone help, please? It's driving me mad!