Well - been working on this all day…but sorted.
I dug deeper into the Register Editing (regedit) suggestions that both the MS Support and a couple of the Youtube folks said. Trouble is that it was a bit of a varying story. One delved into the HKEY_LOCAL_MACHINES and one did the same trick in the HKEY_CURRENT_USER to basically bypass the hardware/systems check. In the end, I did the same change in both and this time it finally loaded the Mixed Reality Portal and I have VR in MSFS again 
I really, really don’t like delving into registries - and with a Windows Media app on a Windows operating system using a Windows Mixed Reality headset - well, to be honest, I don’t see why I should have to.
Anyway, I think it calls for a whisky instead of Horlicks with bedtime only an hour away. Started this at breakfast! But great to be back in VR!