Thanks, zichella1, but I've already tried this method - configuring for Windows 10 as it was the latest option I could configure for.
I've also uninstalled and re-installed on Steam.
One other thing to note - I was getting the same UAC message for Galactic Civilizations III. I verified files and found lots of missing files. Now that these have been downloaded, it seems to work fine.
So I verified files for Galactic Civilizations I and 1 file was found missing and was reacquired. Steam now tells me "All 1320 files successfully validated" but that UAC's still coming up.
Obviously not a disaster as there's only one more thing to press and as blocky and old-fashioned as it looks, still a great game to play...