Win10 Start menu isn't working after upgrade

I thought I'd been let off lightly with my upgrade from Win7 but today on power-up I clicked on the Start icon and got this:

snip - 2015-12 22nd Critical error Start menu.pngI went to the Microsoft Community thinking they'd be able to help but I was wrong - there are dozens of threads there. One was started on the 5th October by the Forum Moderator and gave some fixes. But they haven't worked for the 175 users who've (so far) filled 16 pages with complaints.