OK, I’ve finally got somewhere with the Mavic Pro NFZ removal 
Got speaking to a guy by the name of Qoncussion who pointed me in the right direction and highlighted what I’d missed.
Sure enough, two parameters needed changing in order to completely disable the NFZ in the Mavic Pro:
g_config_airport_limit_cfg_cfg_disable_airport_fly_limit = 1
g_config_airport_limit_cfg_cfg_limit_data = 20250910
That was it, changed those two parameters (to those values) and I was off and running!!! (well, flying)
So you may recall, when I tried to take off from within a NFZ I was told in no uncertain terms, “Cannot take off”. I had a red tail light on the Mavic and I couldn’t even start the rotors:
Then the following day (today), after changing those two parameters I headed back to the exact same place.
You can see from the map that I’m right inside the NFZ (I wanted to be sure!) yet this time, instead of the red warning message in the GO4 app, I’m greeted with a shiny green “Ready to Go (GPS)” message and a green tail light:
Sure enough, it let me fly without an issue.
There was a warning message in the top bar that didn’t go away, but there were no popups, no restrictions and nothing to interrupt my flight. I went out beyond 30m distance and 30m height just to make sure there were no other restrictions. All good!
So there we have it, the .700 firmware now works perfectly with the latest GO4 app and no NFZ issues 
And I can now take off from my back garden without being driven mad by irrelevant warnings 