"incorrect mount option was specified" when mounting krb5p nfs4 partition on Ubuntu

I have a NFS4 share running with krb5p.

I have no problems accessing it from CentOS clients, all that is required is:

This works great on CentOS, I've setup a dozen client hosts so far that way. However on Ubuntu, I get :
mount.nfs4: an incorrect mount option was specified

I think the Ubuntu error is to do with nfs-secure.service however there seems to be no equivalent on Ubuntu that gets installed with NFS client ? (I am using Ubuntu 16.04.5 LTS).

UPDATE:

I have tried:
systemctl enable rpc-gssd.service && systemctl start rpc-gssd.service

That launches OK:

But Ubuntu just hangs when trying to mount ?