Ubuntu 11.04 Alpha1 (Natty Narwhal) - which I was trialling but gave up on - is now a little closer to working!
Having seen a post on Web Upd8 regarding testing Unity in VirtualBox, I found the graphics issue there is indeed fixed and Alpha 1 runs as intended with Unity as default desktop. Sadly its still the crippled and incomplete version, but it is on Compiz and it does run in a VM.
Still more sadly, I put the Pentium-4 physcial machine back onto 11.04 and although there's a recognition of restricted drivers for the NVidia 4800, there's no sign of Unity. Nada. Zip...
I don't know why it isn't working, there's no customary message stating the lack of 3-D support, I just get the standard Gnome desktop and nothing else. Granted it's running faster than 10.10, which is a surprise. Mind you, every item in my Gnome panel randomly crashes and needs a reload (sometimes every one of the eight items crash together on startup), so all is not entirely stable.
I shall continue digging and see if I can't get some Unity with the Narwhal... RC