Finally got it installed. This is what did it...
According to this https://www.computerworld.com/article/3199077/microsoft-windows/windows-10-a-guide-to-the-updates.html
The update progression from OS Build 16299.68 is via 16299.98 to 16299.125
So I manually updated using KB4051963 to OS Build 16299.98 and then manually installed KB4054517.
The router was turned off to stop the automatic updater which was trying to butt in.
Took a while but there were no further problems.
Don't know why KB4051963 was missing.
I would expect Cumulative updates to be designed to contain all previous update files. If that is the case then KB4054517 must be incomplete and is more like a "Delta" update.