The Mirror Tools on the toolbar have disappeared, when I was adding other items to include on the toolbar.
I have tried adding them back but they are always the wrong ones without the drop down menu to
hide / lock . I tried uninstalling the software but it was still not there after reinstalling.
Any ideas ?

Go to Settings -> Configure Toolbar, and add "Mirror" to a toolbar. Make sure you select "Mirror" not "Mirror View" or "Mirror Horizontally" or something like that.