Brakedwell:
Going back to the two excellent Hastings photographs that you posted a few days ago alongside one of which you stated that "GA was the 99 Sqn code". Actually, GAW and GAY were the "last three" of the international radio callsign which, in this case, were MOGAW (WD477) and MOGAY (WJ332). It went something like this:
M = British Military
O = Transport Command
G = Aircraft Type (Hastings) J was also used
A = Base/Unit (Lyneham)
Y = Aircraft individual letter
I shall attempt to attach a rather
poor shot of WJ332 showing the owner as being 53 Sqn in the diamond on the fin.