DaveReidUK,
Why so? You can construct a GC between any two points on the globe, including any pair of points on that arc.
We only know that the aircraft was equidistant from the satellite at two specific points in time.
The reply was to a comment that the aircraft could be flying rather than stopped, and still remain equidistant from the satellite (if it ever was) - but that only works if it flies so that it's on the red arc at two points an hour apart. The red arc is not great circle. The coincidence for it to fly for an hour on a route that links two points on an arc equidistant from the satellite seems unlikely.
Without having information about the other satellite details though, it remains an enigma, and the only thing that's available is the red arc from Inmarsat.