Well the update attempt failed again after I empied the Software Distribution folder. It got to the 3rd reboot at 98%, then said it had encountered "an error" and restored me back to 25300. I didn't see any error code, so I scanned the CBS log and found error 0x80070002. I haven't figured out whether I was looking at the right place or how to proceed, so I'll keep digging. Maybe some kind soul will offer a bit of helpful advice. In the mean time, I'll do some more head scratching.
You could DL install and run SetupDiag JD
https://learn.microsoft.com/en-us/windows/deployment/upgrade/setupdiag
The resulting text file should point to an error/code.
Davy.