Here's a starter, from the Air Force List of 1938;
A C Stewart Acting Pilot Officer; Seniority 5/9/1937. With 57 Sqn at Upper Heyford.
1939 List doesn't give a breakdown of sqn members (oddly enough....), but he was a Pilot Officer with seniority of 12/7/1938
1940 List - he's a Flying officer, seniority 12/2/1940. Once again, the AF List (for obvious reasons) does not provide the handy list of names of who is with what Sqn.
Quick scan of the other bits of the 39 & 40 Lists suggests that he wasn't an assistant to an AOC or anything of that sort. Perhaps the 57 Sqn F540 from the PRO might provide info of where he went to next??