Mike- You did not say what squadron he was in but you mention 1 Sq so I assume that is it.
British Aviation Squadron Markings of World War I has a color plate of an S.E.5a (B8508) in the post-22 March 1918 color scheme plus two photos of the same S.E.5a (C1835) with two different C Flight letters on the aft fuselage. Markings consisted of a white ring aft of the fuselage cockade and a letter for flight and pilot aft of that. A Flight used A-F, B Flight used G, H, J, K, L, and M, and C Flight used N, P, Q, R, S, T.
Colin Owers' collection of S.E.5a photos only contains the line-up of 1 Sq S.E.5a aircraft and personnel you mention, which I will e-mail it to you. The book above has another shot of the line-up from a slightly different angle so the first S.E.5a on the left, C1114, is visible. C1102 does not appear in the line-up because the photo was taken in autumn 1918 after C1102 was shot down.
Taz
Terry Phillips