It seems that this error in the KB5007651 is reported differently in my Event Viewer.
Whilst the error listed in the Windows Update failure is 0x80070643, when I look at the Event Viewer errors, the source is shown as
Event 20, WindowsUpdateClient, and the error in the General description is given as:-
"Installation Failure Windows failed to install the following update with error 0x8024200B: Update to Windows Security Platform - KB5007651 (Version 1.0.2109.27002)".
Which is it? More to the point, what's the cause, and what's the fix?