I did some more work on this today as follows:-
1) Unistalled each app that I had installed after the clean boot (after each uninstall, I rechecked the status with Memory Integrity). Result, even after uninstalling all the apps, the situation remained unchanged.
2) Reset the laptop again (cloud reset option), but didn't install any of my own apps.
3) Rechecked the Memory Integrity status. Result: Although there was no warning on the Windows Defender icon at the bottom of the screen, on checking Memory Integrity via the Core Isolation route, it was apparent that Memory Integrity was off and still wouldn't turn on for the same reasons as I reported above. The photos I took of the computer screen show the status after reinstalling Windows.
4) After carrying out the above checks, I restored my laptop using the latest Macrium Reflect disc image, which I made yesterday.
So in conclusion, this appears to be something over which I have no control, so I'm not going to worry about it any more. I've reinstalled Avast Premium Security, which should provide extra protection for much of the bad stuff out there (hopefully).
