Use of SBF GPS causing uncontrolled flyaway but only on PH2.1

This is a repost from the Pixhawk 2.1 hardware section as the code for the SBF in the latest FW is done by @Michael_Oborne as far as I know.

So this is a heads up and a general question :slight_smile:
What could possibly make an SBF GPS unworkable on a Pixhawk 2.1 but OK on a Pixhawk 1???

Link to post in Hardware:Pixhawk2.1