There are a number of files at the National Archives describing development of these devices. There were constant experiments to improve flares throughout the war and it is likely that the TNA files describe this.
Five files in particular look promising: AVIA 15/90, 91, 92, 93 and 94, titled ...
RECONNAISSANCE AND OBSERVATION (Code 43): Submarine marker flares: design and trials
...spanning 1939 to 1945.
Hope this helps,
Bruce