Since using Teamviewer 12 trying to sort stuff out with the Steinberg support I now have a black desktop and in addition I can no longer separate my mix console and project events window onto my two separate monitor screens but only have a squashed bunch of faders beneath the main project window. Anyone got any ideas (constructive) please?

Have you uninstalled Teamviewer? Do you have a system restore point from before you installed Teamviewer that you can revert to, or system backups?