Apologies for the late response... i have been away over Christmas and new year (with no issues getting online without the Superhub 2ac!).
I have contacted Virgin Media support since my return and was told that the issue is with the Switch and that i would need to contact Virgin Media Gadget Rescue (which is a chargeable service). When asked if i could swap the router out, was informed that they could only do this if there was a fault with the router, and to diagnose a fault i would have to pay... and around in circles we went. I could get a SH3, but i would have to pay for the upgrade. I'm not exactly happy with the outcome!
I didn't want to threaten to leave as that seems a little excessive. I may try again at some point. Does any from Virgin monitor these community posts, so i can communicate direct with them to try and resolve?
If i have to pay for anything i will be buying my own router...
So i took the plunge and purchased a new router on the recommendation of a couple of review websites, an ASUS RT-AC66U.
Long story short NAT Type is still an F... how is this the case. Have i literally purchased the only other router in existence that causes the the same outcome as the SH2 AC? I have no idea what to do now... Surely there is an issue with Nintendo as well being overly verbose with what ports it actually needs to work?
If it's in modem mode and the Switch is now connected to the ASUS router it should "just work" without you needing to do any manual port forwarding or DMZ. If you've already added any port forwards/DMZ for the Switch try removing it and then give the Switch network test another go.
So i made a couple of changes... re enabled router mode on the superhub and made sure i had cleared out any rules that i had created but not used and also deleted the static IP i had assigned to it. (it shouldn't be using any of this in modem mode but did it anyways).
I then switched the superhub back to modem mode, deleted all the network settings on the switch and reconnected.