After some research online, it seems I wasn't alone to have a Windows Update that never progressed beyond 20% and always failed. Moreover, it seemed to be holding up other updates, such as the latest Cumulative patches for June 2021, as shown below:
I went through various tests, including the advice here, resetting the Windows Update components and running through the usual restarts, all to no avail. The error persisted.
But that link did point the way to a solution - upgrading Windows to the very latest supported branch manually by accessing the Update Assistant on Microsoft's site. This examines your set-up and offers a one button upgrade-in-place. The hope for me was that it would overlay any problematic system files that were blocking the original update.
And... it worked beautifully. There were no prompts as to whether it should affect all currently installed applications or my data - it just proceeded to grab a new version of the core of Windows 10's latest branch and install it 'in place'. Everything else did indeed carry on working and nothing got deleted.
Most importantly, Windows Update now shows that nothing is waiting or in an error state.
Which is uneventful enough (and off topic enough) that this should remain a short 'Flow' story. But if you have any Windows laptops or ultra-mobile 2-in1s that are having troubles updating then give the Windows Update Assistant, https://www.microsoft.com/en-us/software-download/windows10 a try. It seems an all-purpose URL of note.