Following further investigation, I found that part of my problem had been the lack of a module; Follow Me (not one I've ever had cause to use over the years). Anyway activating this caused another error with the php.
Long story short, I'm at a brick wall now of;
Error (client) illegal state key onOffState defined by plugin -- native state keys cannot be overridden (ignoring)
I don't seem to be able to find the underlying problem. As such I'll just have to put it down to an incompatibility with RestAPI 15 unless anyone else can confirm they've had it working.
A shame, what I was really after was to have call forwarding based on Geo location, handled by Indigo.