I received contact from the mods and they referred me to the VM tech team to look deeper into this. Sriram, their engineer, wanted to figure it out. I spent quite a while reconfiguring my network to replicate the problem for him yesterday.
The problem manifested the moment I plugged in my previously used unmanaged switch.
Sriram pushed a new firmware update and tested several settings. Eventually he was able to get it working. I believe he removed the VM configuration that is applied to the router.
He then rolled back the firmware, which obvious broke things again. Finally he removed the configuration and managed to get it working with the firmware update that broke it in the first place.
Now, here's the comical element. After working around the issue at my own expense and spending a lot of my own time helping VM diagnose the issue, and after being promised an escalation by the complaints team and not hearing from for over a week - after being promised a response in 72hrs, I chased them again today. They have increased their compensation offer to £46, still blaming my equipment. Yet, as recent as yesterday I was assisting them to help them diagnose their own problem.
I'd find it funny if I'd not already invested so much into their problem already.