The March is Walton's the introduction to it is Arnold's...

Here are the parts of the manuscript in Arnold's hand (the rest is in Walton's handwriting)

5M1 "Pilot's Run" 0'59"
12M1 "Interior of Heinkel" 1'15"
13M1 "Battle in the Air" (pages 1-15 in Walton's hand , pages 16-24 in Arnold's)
14M1, 14M2 (no title) 1'31"
14M3 March Introduction 2'14"
14M3A alternative version to 14M3 2'11"

(from Alan Poulton's book)

I agree that at  FSM  they occasionally take film music too seriously. I think there are clear cases of "take the money and run" for every composer in films. From Shostakovich to Rozsa.