I've managed to get this to stand after hours of trial and error and swaping back and forth on ERL firmware versions.
In the end I had to manually specify the left and right subnet in the ipsec.conf file on the USG.
This allowed the VPN to stand up, but isn't yet passing traffic which is my next task.
Annoyingly this means it will break anytime a provsion is done, is there anyway I can load this into the json config ?