Just had a nosey around military stock number codes and this number came up.
RN Ensign NSN 8345-99-571-3291
The last 7 numbers are the same as the ensign above. The 'NSN' prefix stands for 'Nato Stock Number', ergo the flag cannot be pre April 1949 at the earliest, and is not of WW2 vintage. The brass fittings on the lanyard are known as 'Plain Inglefield Swivel Fittings'.
Shiver me timbers, Ned.