|So far so good...|
No thanks to some display troubles I've been having, I've somehow managed to mess up my Compiz settings (wallpaper with no Dash, no indicators, nothing). So... it's a bit early but I've upgraded to Ubuntu 14.10 after downloading and burning the latest daily image on my laptop which was, for better or for worse, running Windows 8.1.
Unfortunately, that didn't fix the problem of the missing Unity desktop even though the upgrade said only data would be left intact and settings would be removed. Or, maybe I've misunderstood the wording.
Anyway, I've luckily managed to get Unity back without needing a clean installation. So now I'm back to "normal".
The flickering's gone and so far no white/black/grey screens have appeared. We'll have to wait and see.
But things aren't all back to normal. My normal screen resolution (1366 x 768) has now shrunk to 1024 x 768. I also appear to be on Unity 2D mode. I'm guessing my GPU's about ready to retire.
And to think the Sapphire HD 7750 isn't that old a card. Oh well, time to get a replacement. I had hoped it'd last a few generations at least.