Thanks for getting back to me.
That evening I went out and noticed the water company had dug a hole next to our street cabinet- coincidence? Not sure!
During the night VM recognised there was an issue in my area (although the status check didn't seem to think so) so they cancelled my technician and said the issue would be looked at that morning. Shortly after that I received another message to say the issue had been resolved. However, we were still offline and rebooting the router still didn't help. So I was annoyed that the technician had been cancelled before it had been confirmed that the issue was sorted.
I used the app again to run a test on the kit and it recognised that there was an intermittent connection issue, it stated that Virgin were 'looking into it' and to wait 24 hours. During that 24 hours I tried a couple more reboots where the router flip-flopped between identities (VM/UPC), and at the end of the 24 hours it still wasn't working. Running another test, the app triggered a factory reset of the router. I was glad that the router was visible on the VM network, and could see this in progress when the light changed from solid white to flashing green without my having to do anything manually. The factory reset seems to have worked as we were back online on Saturday morning, and the router is still calling itself 'UPC'!