Same again, here is wiki's list of RAF Squadron Codes (as distinct from individual aircraft ones that completed the three letter code, usually painted either side of the roundel):-
https://en.wikipedia.org/wiki/List_o...squadron_codes
X on its own doesn't seem to figure for any of the squadrons in Danny's list, being mainly allocated to FTS's or AFTS's (could it be one of the latter?).
However if it were followed by another letter, cropped out of your picture, then the field is wide open and you may take your pick. Unfortunately none of them seem to then relate to a Squadron in Danny's list in 1942. If the picture's date is different to that, and there might be a second letter after the X, then it is a matter of matching a code to Danny's list.
If the X is clearly on its own, my guess is that it was aircraft X-Ray of a yet unidentified squadron...