OK, just took her for a first flight using GS following adding the 2.4GHz Data Link. Apart from the fact that the position of the Phantom was reporting an altitude of 8m when on the ground everything worked perfectly. I did a simple 3 way point route and she took off after pressing Go and then flew the route just perfectly. I auto landed her twice using the feature in the GS app but on the second attempt she stayed hovering about 9" above the ground. So, she is interpreting the route and waypoints perfectly and carrying out her instructions exactly as entered. So a huge success.
Now I just need to work out why she was reporting an incorrect altitude when on the ground. No idea if this is to do with the mod or not. No more time today unfortunately due to guests arriving but this week I will do some more testing both with the mod and GS and also after disconnecting the Data Link and check what altitude the DJI Vision app reports. I did do a very poor video with sone holding iPhone and will post a link to it once I have uploaded to PC and Vimeo.
[edit] just read that the altitude reported in GS is actually height above or below sea level. So appears it was correct and me just reading the app incorrectly. So all good then