Looks like I have got it sorted now. My error was when I setup a new NAS Client I had entered the IP address of the physical Data Mover NDMP interface, which worked fine if you are doing NDMP backups, which I was. I setup the NAS client again and this time gave it the FQDN of the CIF server I had setup on the VNX, it then prompted for me to enter details of an account to use. Then as soon as I ran an Archive job, bingo everything got stubbed out as expected :-)
So my error/misinterpretation of the documentation.
Thanks for showing an interest.