Cumulative update failures - Aug & Sep

Both the August and September cumulative updates have failed on my XPS13 (9360) - they both fail at 0% with error code 0x800f0900.  

The updates are as follows:

• 2020-08 Cumulative Update for Windows 10 Version 2004 for x64-based Systems (KB4566782)
• 2020-09 Cumulative Update for Windows 10 Version 2004 for x64-based Systems (KB4571756)

I've followed all of the suggestions here for resolving the problem, but with no success.  Dell SupportAssist runs fine, as does Dell Update, and the Windows Update troubleshooter can't identify the problem.  I had hoped the September update would solve the problem which surfaced in August, but it seems not.

I can't go back to a previous version of windows as the computer was updated more than 10 days ago, and I'm loathed to do a reset as I'll lose all the Dell apps and others - any suggestions?