Years ago I read a Biography by the excellent John Pearson. I see there's a new one due next year and perhaps it will reveal more about his war time work.
"Though McCormick generally accounts for the biographical factors in Fleming's childhood (his father's death and his mother's strong nurturing) and his restless youth (studies in Germany and international reporting for Reuters), Fleming seems incomplete and distant by the time he has his crucial experience in naval intelligence in WW II under the code name ``17F.'' With these espionage operations still partially classified and permanently obscure, McCormick plays down Fleming's adventurism, with the exception of the bizarre case of Rudolph Hess. In the book's most mysterious chapter (which digs into Fleming's interest in the occult), McCormick places Fleming murkily in the plot that, by playing on Hess's superstitions and interest in astrology, lured the Nazi to England with the false promise of negotiating peace."