Bingo. Why didn't I think of that?
You can't set a gateway through the web UI but you can set a static route through the telnet interface. So I've added a route to my address space via the USG and can now reach the Vigor 130 web interface from the LAN.
Next I need to work out how to set a static IP on the WAN interface on the USG without having to do it at the command line. The USG checks in with the controller periodically so about 5-10 minutes after setting the IP address in teh CLI it'll be removed.
The USG uses the same CLI structure as the ERL so I just need to to generate a
config.gateway.json file and put it in the relevant place on the controller. I also need to check if the static route on the Vigor 130 is persistent or not so I'll reboot it later and see.