Ok chaps so here's the issue.
I successfully set up my RT-AC68u ( running Merlin)with my Virmin superhub to by sticking it in modem mode and all 'was' working fine.
Now i've setup a VPN and all the clients work and connect fine but when connected via VPN through the clients I get an error message in Plex that it's not viewable outside my network due to Dual NAT.
I can understand why as my WAN IP is my orignal Virmin IP and the VPN one is different.
Does anyone know which settings need changing (and to what) to get my plex viewable outside my home network?