kiwimatt: Thought I would post this in case anyone else with this modem router finds this thread.
There is no need to throw the router out as has been suggested to me! In fact we are finding its performance really good. The fix above is for a different router and doesn't apply.
Never managed to get it running with Xnet's VFX. Tried on Open VFX using various SIP clients and back on standard VFX using a SPA112. Logged calls with Xnet help desk. Also logged call with Dlink. Dlink were great - a very on to it tech called me multiple times and ran through a number of tests over the phone demonstrating the unit working well with other voip providers. The Xnet helpdesk escalated my call but after over a week a call back has not been received.
Long story short I switched to 2talk's voip service and couldn't be happier. Super simple to set up, loads of features, great bundled calls.
That's pretty much confirmed the router was at fault.