Workaround / temporary fix
If you have a Virgin Media Business connection with a Static WAN IP, delivered over COAX - then to be able to give you that static address - Virgin create a GRE tunnel between the Hitron router and their head end.
Nb. this is not the same as the sticky IP addresses VMB tend to give out.
The problem appears to be that following a firmware update - the Virgin head end is not provisioning it's half of the GRE tunnel correctly. The Hitron is getting it's half - but the head doesn't think its expecting one (confirmed by the customer service saying "there's no static IP on this customer account" which is incorrect and aligns with the corrupt database issue outlined in the article) - so doesn't set it up.
If you run a traceroute from the web interface on the Hitron - you'll see that the source IP is the static / assigned by GRE address and not the general dynamic DOCISIS one.
No traffic can be routed = no usable internet connection
A factory reset does NOT include wiping the details of the GRE tunnel - so you have to put it into modem mode to get it to forget about the tunnel completely. Then factory reset to make it a router again.
You lose your static IP and any / all custom configuration you might have made - which is dull.... but at least you're back online again..