Well, archive and install didn't work
But I think I've sussed it... without going through all the gory details, it would seem that Shades was the culprit, which I was running as a more convenient way to control two monitors.
The system also starts a
lot faster and generally feels a bit "snappier"... I'll give it a while and try running Shades again, a brief attempt seemed to indicate that quitting it and restarting might have been sufficient.
I suppose having the graphics drivers update whilst it was still running could have got it confused?