found another thread which suggested the problem could be due to the Windows installer having stopped service. This turned out to be the case. The fix was to find and start the Installer before trying the Update (yet) again.
Click Start, type run to get the Run command line, enter services.msc, scroll down to Windows Installer and start the service. Run the update. Worked fine.
Microsoft .NET Framework 4 (Standalone Installer)
The Microsoft .NET Framework 4 redistributable package installs the .NET Framework runtime and associated files that are required to run and develop applications to target the .NET Framework 4. 4/12/2010
Main PC: Asus TUF Gaming 570-Pro (wi-fi) * AMD Ryzen 7 5800X * Noctua NH-D15 * Corsair Vengeance LPX 32GB * Asus TUF Radeon 6800XT * Creative AE-9PE * 2 x Samsung 980 Pro * 7 x WD Gold HDD * Corsair HX 1000 * 1 x Asus DRW-24D5MT * Dell U3010 * Windows 10 x64 *
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Old workhorse PC: * Intel i7 4790K * Noctua NH-D15S * Asus Maximus VII Hero * Corsair Force MP510 480GB M.2 SSD * 32 GB Corsair Dominator Platinum CMD32GX3M4A2133C9 * Sapphire Radeon R9 290 * 3 x Dell U2410 @ Eyefinity 5760 x 1200 * Corsair HX 1000i * 7 x WD Black / Gold HDDs * Creative Soundblaster ZxR * Asus DRW F1ST * Corsair K95 RGB * Corsair M65 PRO RGB * Steelseries 9HD * Coolermaster STC T01 * Edifier S530 * Sennheiser HD598 * Windows 10 x64 *
