I saw very similar issues in customer-environments when vCenter was on 6.7 GA and hosts on 6.7 U1 (unsupported for vSAN but not ESXi), current compatibility matrix also states only 6.7 U2 vCenter is supported for vSAN 6.7 U2 nodes so first thing to check would be that vCenter is on 6.7 U2.
One could also confirm that vCenter/UI is the issue by manually creating a Disk-Group via CLI:
# esxcli vsan storage add -s naa.xxxxxx -d naa.xxxxxxx -d naa.xxxxxxxxxx -d naa.xxxxxxxxxxxx
Thanks - The command does complete successfully and I've completed it on all disk groups on all hosts and I even added the hosts to the vSAN cluster with
esxcli vsan cluster join
Now I have a vSAN datastore which I can use but the vSphere Client thinks vSAN is turned off (select cluster > Configure: Shows "vSAN is Turned off")
So now I'm unable to configure vSAN from the UI.
This is just a lab so I'm happy to start again and see what's causing this.
As I was saying, if you can do it from the CLI then is very likely a vCenter issue.
Can you confirm the vC is 6.7 U2, valid vSAN license is in vSphere inventory, try from both UI Clients, try a different browser etc. .
Thanks for following up, you were spot on there.
I initiated the upgrade of the VC 6.7U2 but it turns out that it hadn't actually completed!
Got it all sorted once that was done.