Sorry @PaulR, didn't mean to chop you off at the knees there bro! Just thought that, typed it! I appreciate you were trying to help.
@tgc have disabled VLC and this and it has cured the problem, but as you correctly point out, this takes us back to …
@PaulR No, that's not much help, it's just describing what should happen. What I'd like to solve is why it isn't doing what it should. Nothing has been reset. Apart from that, as I described in my original post, videos and PowerPoint are unaffected…