After checking
some online posts, I managed to empty some cache files, and it seems that cleared the blockage. I can restart the Mac and the update resumed installation. However, Spotlight now seems problematic and finding older emails in Mail from the search field is impossible.
I'm trying to reindex Spotlight, but not sure how that's going. Really Apple? To reindex Spotlight you have to fiddle with the Privacy pane in System Prefs, and even then you have no visual idea if it's actually reindexing? Who designed Spotlight? A visiting Windows team?
Anyone have any tips on fixing Search in Apple Mail - High Sierra?