No AV software other than Windows Defender; I should have added I disabled it when installing Windows Repair once I started having issues but that made no difference to the result. I have nothing else running that could interfere with the install. Like I said, all other updates I do using the built-in update feature worked fine until this version, when it appeared to update properly but wouldn't run without throwing up that error. That's when I uninstalled it (using Geek Uninstaller) and reinstalled several times trying different strategies each time; it still chucks up this error. I'll try uninstalling it and installing an older build and see how that flies and report back.
Thanks, Dave.