Okay, so first of all I discovered I wasn't seeing any changes I was making to existing modifiers already in the tuning because of a mod conflict.
I've sorted that and I can make changes to the tuning that work in game BUT it won't make use of custom modifiers added to the list. So either the game isn't seeing them or I'm not formatting the code for them correctly.
Some official input on this topic woud be great. I've had some help from another community member to get me this far but that's as far as I can get with their help it seems.