2 issues I encountered.
1) Default gateway. Took me a while to realise that the machine was stuck on the wrong default gateway. Perhaps I plugged in the wrong gateway on setup (don't think so) or perhaps it was simply a default, however I was unable to change it using netconfig and had to edit /etc/sysconfig/network file to correct the gateway. netconfig would accept the inputs ok and everything else I put in there worked ok, just not the default gateway.
2) Once that was sorted, the trunk was shown as registered and I could call in just fine but not call out, I would constantly get all lines are busy (or words to that effect). I had used Tony's instructions to setup the box and when that didn't work I tried Joshp's instructions here.
After finding zimbonz's thread here I deleted the account settings and used his approach. I edited the sip.conf file as per Joshp's instructions then followed Tony's instructions for using the trixbox UI to enter Josh's sip_additional.conf information under PEER Details for the VFX Trunk and viola.
A big thank you to Tony, Josh and zimbonz.