Tmmoris,
Quite right, thank you for the clarification. I was a little lazy in my reading of the previous post.
Nonetheless I have noticed that the iphone gps is temperamental even at ground level e.g. when using a speedometer app. This gives me a few concerns around using it in flight even as a backup. By definition a backup must be infallible!
Given that airbox devices are available very cheaply second hand I suspect this is the way to go for me.
Thanks again.
T.