I just want to get back on track with Cumulative Updates

Hi, as I understand it there are two issues:
1. that you can't complete Windows update
2. that what appears to be an attempt at an in-place upgrade repair failed

The first you might be able to correct with
Reset Windows Update in Windows 10 | Windows 10 Tutorials

The second- we're awaiting the screenshot of your partitions using Minitool Partition Wizard (free) expanding all columns as necessary so all text is visible.

As regards that link, the key question is which partition is marked as active, rather than it being a dual boot config or not. The latter was the scenario, the former, the issue.

That lines up (I think) with what you say: After attempting to boot into it I discovered that it actually altered the boot on my system disk! Seemed to have installed GRUB.

Key sentences:
Changing the active partition to the Windows boot partition would probably have the same effect.
To circumvent this I changed the BIOS settings on my PC to boot directly from the Windows hard drive

1. Try the tutorial linked above.

If that doesn't help, try Tweaking.com's free cautious repair tool, and select only repair 16 (Windows updates).

Then try the Windows Update Troubleshooter as post the report.

Then try running Windows Update and post the outcome.

2. Yours is a legacy MBR configuration. Nothing immediately strikes me about the partition layout, except that any upgrade attempt should be made without the USB attached, unless the USB is itself a bootable Win 10 medium which you are using for the upgrade.

However, I am not familiar with Grub, and hope perhaps someone else can advise on that.

You are correct about using an in-place upgrade repair install- it replaces all system files, re-sets some registry settings to default, makes sure services are running as default etc, so can resolve a wide range of system (not per user) problems.

As for your comment about AVG and McAfee, I've never heard of that in relation to Minitool PW. Perhaps you got the installer from a dubious source.
Best Free Partition Manager for Windows | MiniTool Partition Wizard Free

I've got it fixed!
The tool that did it was the repair kit from tweaking.com. But there was trouble along the way ...
Tweaking was quite stern in their admonishment to run from Safe Mode when using the repair tools.
Problem was that I couldn't get into Safe Mode.
Tried msconfig, but in the text box that shows the boot record to choose, I had nothing - the box was empty (screenshot below is after I fixed that problem). Therefore all the options, incl. Safe Boot, were disabled.

Eventually I found out about bcdedit. I could use it to enable the legacy boot mode where F8 key would bring me into Safe Mode (apparently F8 is often disabled with UEFI systems running Win 10).
But bcdedit told me The boot configuration data store could not be opened. Access is denied.
Researching this, I found out that I would need to repair my MBR and that Macrium Reflect could do it. Fortunately I already had a USB drive with Macrium. (I also found out that most people with an empty boot data store cannot even boot their system, but I had no problem with that).
Macrium repairs MBR and now bcdedit and msconfig show me a valid boot selection (as in the screenshot above).
So I get into Safe Mode, run tweaking.com tool for Windows Update repairs and a short while later I finally have KB4338819 (July 2018) installed!

Thank you all for your advice. I've now added some great tools to my repair kit. Too bad I needed a repair kit.