Just wanted to update and share.
I have finally managed on Nvidia to get 23.976
4k RGB 10bit..yep, not 12, but 10 (both SDR and HDR) (confirmed also by my AV receiver on-screen info).
I discovered that using the display port (which is 1.4 on the 1080), and using the Club 3D CAC 1080 Display Port 1.4 HDMI 2.0 A (B) Hdr Active displayport to hdmi adapter, i can now choose between 8, 10 and 12bit in NVC @ 4k (10 and 12bit upto 30hz).
Using the adapter also puts the driver into a slightly different mode, in as far as Set Content Type is no longer available etc. Also, the white point scale and crispness is different than straight hdmi (which I prefer over straight hdmi). Some of the Nvidia drivers will only report upto 30hz being available using the adapter, but 397.76 displays all modes upto 60hz. The trade off is, as with some previous drivers, you cannot set up a custom res at 10bit (defaults to 8), so thats a down side - I guess it's about which trade offs you can live with.
At least on my set up, the colours seem more faithful, and no more over blown colours in HDR mode.
I know there is a lot of debate about 8bit with dithering etc, but, just wanted to share.
Update:
Ok, maybe i'm not doing the test correctly somehow, but using the 4.4.4 Chroma test image (link
https://linustechtips.com/main/uploa...8c8d22f67e.png ) it also appears to pass the test at 23hz....?