Sophos SG135 WAN side firewall configuration new problem creating a Vlan interface

I have a few subnets and have been trying to bring some discipline to my firewall configuration.  Because the system is live I am trying to create a new interface Test_Network on a VALN using my private connection on the UTM, in this case eth5.
I have given the interface a test ip of 10.99.99.1 and not enabled Dynamic IPv4 or IPv4 Default Gateway but once saved the interface does not appear on the list of existing interfaces.  In fact I have no idea where it goes and the only way to get it back is to use the Find box which brings gives my the interface but is shown DOWN.  If I enable it suing the switch and then disable it the interface is then shown UP but goes DOWN if I enable it again.
Out of my depth here and need some help please. 
I have some other settings which may be relevant, I have a failover connection to a slower internet connection and have started on QoS to try and sort out load sharing because some guests have been hogging the bandwidth but at present this is not the critical issue.
I can share logs if it would help.