The song evolved allot.
I like Doing Alright best done live at Earl's Court 77 show.
That's the ultimate version of this song as far as I am concerned.
I'd say absolutely definitely written on the guitar.
The fact that the main piano part translates (assuming for a second that it was written on piano) so perfectly on to the guitar is just too much of a coincidence. The opening riff and chord sequence used are absolutely natural on the guitar.
It's in E, the mother of all guitar keys and the most guitar-friendly, and the F#m11/E and Emaj7 chords played are standard chords/voicings on the guitar which any guitarist easily slips into when fiddling around.
I think Bri's first piano-based song was Dear Friends.