The Bowyer/Rawlings ‘Squadron Codes 1937-56’ lists 248 as WR (39-43 ish) and then DM until 46 as shown in this link, which also cites QK prior to 39:
http://www.rafweb.org/Sqn246-250.htm

But the MOD shows the code as RK in their web history on the squadron as does this other listing:

http://www.raf.mod.uk/history_old/h248.html
http://www.btinternet.com/~lee_mail/rafcodes.html

Bowyer & Rawlings attribute RK to 10 OTU .

Confused. I tend to believe Bowyer/Rawlings.