Hi Richard,
I read your post late last night but my brain was too tired to think straight. Can you post a screenshot of your PilotAware Home and Configure Screens, so we can see exactly what is showing?
My first thoughts were that this was a homebuild, either Pi1B (which won't run Mode C/S), or with no 'Bridge' or your licence key is not properly entered or saved, but from your post, these all sound unlikely.
You say the home page is showing 6 satellites, but is it getting a good position fix? This can take quite a while first time out of the box. Without an accurate position fix the P3i transmitter won't run, as it would be sending incorrect position data. The Home screen will show this.
Finally, you say you are using an iPad power supply with a good quality cable. Is the power supply definitely rated at at least 2 amps, and is the cable the one supplied by PilotAware?
Regards
Peter