Somewhat ironically, I found that one of the Windows 11 requirements was actually turned off in my installation.
I had to enable Intel (VMX) Virtualisation Technology in bios in order for some of the device security options to be turned on. That in turn switched on in Windows 11 "Core Isolation" in Device Security, when there is a Memory Integrity Switch that prevents malicious code being inserted into processes.
To quote MS, "Memory integrity, also known as Hypervisor-protected Code Integrity (HVCI) is a Windows security feature that makes it difficult for malicious programs to use low-level drivers to hijack your computer".
This refused to switch on, listing three drivers as non-compliant. However, as they related to redundant features (a CD burner in an very old mpg video software suite and an equally old Belkin usb video input device), I could then, thanks to Google, command-line delete the services the drivers were part of. The Memory Integrity switch then worked. The Hardware Security also changed to Enhanced.
No idea how important this is in real world situations but at least it is all working.