Thanks pndwalThe point is that two (or more) Apps / Managers conflict.
Please locate and remove any renamed Magisk App from device settings, Apps, App info (or equivalent). If you forgot names you may have chosen, go through the whole list... likely it's default choice, 'Settings'!
Nb. Magisk general settings (hidelist, Superuser etc) & modules survive as they are stored in /data/adb instead of in app data.
If that doesn't fix conflict (App says 'Installed N/A' etc), you may need to uninstall / reinstall unhidden Magisk App also.
While updating (App) with App hidden, especially between consecutive builds, is supposed to work (coming from old builds to post 22.0 is an exception), in practice the new App has failed to replace the old one some 50% of the time when not first un-hidden. It is therefore a good practice to 'Restore the Magisk app' before updating. PW
I didn't realise it would be as simple as removing the renamed Magisk. I did that and installed the version of MM that I originally flashed with. It detected Magisk, root was still working but I still wasn't able to access Magisk settings and no su prompts were being generated for any new root apps. I then tried a newer version which I had stored on my device and everything is now working perfectly.
Not gonna update Magisk as I'm still going to flash with 11 over the next few days but pleased it's now working as it should.
Appreciate the help.