So what I hadn't realised was that I needed to be an actual user within the B2C AD.
I added myself as a new guest user
Then attempted to access it the B2C
I was then given the opportunity to set up my Authenticator Account for B2C access.
Once I was able to log in - I was able to access the full B2C as an admin. Nice.
I now have 3 separate accounts for dev, uat and prod.
The error I was getting without access was a little confusing, but now realise that this is the default error when you have no access to a resource.
hope this helps