After the war he returned to Cambridge University where he was a history lecturer from 1921 to 1928. This was followed by a brief spell as a theatre producer and actor. He also advised the Government Code and Cypher School (GCCS) on recruiting academics. Josh Cooper, told Michael Smith, the author of Station X: The Codebreakers of Bletchley Park (1998): "He (Alastair Denniston) dined at several high tables in Oxford and Cambridge and came home with promises from a number of dons to attend a territorial training course. It would be hard to exaggerate the importance of this course for the future development of GCCS. Not only had Denniston brought in scholars of the humanities of the type of many of his own permanent staff, but he had also invited mathematicians of a somewhat different type who were especially attracted by the Enigma problem." (4)

Francis Harry Hinsley later claimed: "Denniston... recruited the wartime staff from the universities with visits there in 1937 and 1938 (also 1939 when he recruited me and 20 other undergraduates within two months of the outbreak of war). I believe this was a major contribution to the wartime successes - going to the right places and choosing the right people showed great foresight." (5) According to codebreaker, Mavis Batey, the mathematician, Alan Turing, went to one of the first of the training courses on codes and ciphers at Broadway Buildings. Turing was put on Denniston's "emergency list" for call up in event of war and was invited to attend meetings being held by top codebreaker, Alfred Dilwyn Knox to "hear about progress with Enigma, which immediately interested him... unusually, considering Denniston's paranoia about secrecy, it is said that Turing was even allowed" to take away important documents back to the university. (6)

On the outbreak of the Second World War a special unit of the GCCS was established at Bletchley Park. This was selected because it was more or less equidistant from Oxford University and Cambridge University and the Foreign Office believed that university staff made the best cryptographers. The house itself was a large Victorian Tudor-Gothic mansion, whose ample grounds sloped down to the railway station. Lodgings had to be found for the cryptographers in the surrounding villages. (7) Birch was billeted in the Duncombe Arms at Great Brickhill.

Barbara Abernethy recalled that Birch was a popular figure at Bletchley Park: "He (Birch) was a great person. I knitted him a blue balaclava helmet which he wore throughout the war. He was billeted in the Duncombe Arms at Great Brickhill. They had a lot of dons there, Gordon Welchman, Patrick Wilkinson. It was full of dons all the time. All of them having such a jolly time that they called it the Drunken Arns." (8) Penelope Fitzgerald described him as "a many-sided human being - a rather dull historian, an acceptable drinking companion, a mysterious private personality, a brilliant talker and a born actor". (9)

Alastair Denniston appointed Frank Birch as head of the German subsection of the naval section. Frank Birch was based in Hut 4. He told Francis Harry Hinsley that they were trying to read intercepted German naval messages. "The code used by the Germans had not yet been broken. That being the case, Hinsley was to do his best to find out as much as he could from the information they did have about these messages. It was quickly apparent that there was not much evidence to go on. There was the date of the messages, their time of origin and their time of interception, and the radio frequency used by the German morse code operators. Sometimes Hinsley would be told where the messages came from, information which had been gleaned using the Royal Navy's direction finding service." Hinsley became involved with what became known as "traffic analysis". This was defined as "looking at all the evidence relating to enciphered messages which could not be read, and reaching a conclusion on what the enemy was doing." (10)

Frank Birch clashed with two of the leading codebreakers, Alan Turing and Peter Twinn but was popular with junior members of the GCCS. Ralph Erskine admits that "although he was not a born leader, and at times had a heavy-handed managerial style, which was an unwise approach to GCCS's free spirits, he cared deeply about his staff and was highly popular with the junior members of his section in consequence". (11) Birch complained in August 1940: "Turing and Twinn are brilliant, but like many brilliant people, they are not practical. They are untidy, they lose things, they can't copy out right, and they dither between theory and cribbing. Nor have they the determination of practical men." Birch was concerned that Turing and Twinn were not making the most of the suggested cribs he and his team were passing on to them. He even suggested that if Turing and Twinn had used this material correctly we "might have won the war by now". Birch went on to argue: "Turing and Twinn are like people waiting for a miracle, without believing in miracles." (12)

Frank Birch became increasingly dissatisfied with Alastair Denniston, who he believed lacked the drive to press for enough deciphering machines (known as bombes). Ralph Erskine argues that Birch's views were a major factor in Denniston's replacement by Commander Edward W. Travis in February 1942. Birch was responsible for all naval crypt analysis (except Enigma) and for translating and analysing the decrypted signals of the German, Italian, Japanese, French, and Spanish navies. (13)

It has been argued by Birch that the success of this work played an important role in winning the war: "British resources were so meague that even with all the information in the world only moderate immunity could have been obtained. By March 1943, when Special Intelligence was coming along strong, though not yet at full strength, the Germans had become incapable of reading our stuff and in the great showdown of that month the U-boats were, as a result of Special Intelligence, driven off the convoy routes for six months." (14)

Frank Birch, who was head of the historical section of the post-war Government Communications Headquarters (GCHQ) after the war, died on 14th February 1956 at his home in Knightsbridge, London.

(1) Ralph Erskine, Oxford Dictionary of National Biography (2004-2014)

(2) Alastair Denniston, Room 40: 1914-15 (1919)

(3) Michael Smith, Station X: The Codebreakers of Bletchley Park (1998) page 11

(4) Francis Harry Hinsley, quoted by Robin Denniston, the author of Thirty Secret Years (2007) page 24

(5) Michael Smith, Station X: The Codebreakers of Bletchley Park (1998) page 16

(6) Mavis Batey, Dilly: The Man Who Broke Enigmas (2009) page 71

(7) Penelope Fitzgerald, The Knox Brothers (2002) page 228-229

(8) Barbara Abernethy, quoted by Michael Smith, the author of Station X: The Codebreakers of Bletchley Park (1998) page 26

(9) Penelope Fitzgerald, The Knox Brothers (2002) page 93

(10) Hugh Sebag-Montefiore, Enigma: The Battle For The Code (2004) pages 55

(11) Ralph Erskine, Oxford Dictionary of National Biography (2004-2014)

(12) Frank Birch, letter to Commander Edward W. Travis (21st August, 1940)

(13) Ralph Erskine, Oxford Dictionary of National Biography (2004-2014)

(14) Frank Birch, quoted by Michael Smith, the author of Station X: The Codebreakers of Bletchley Park (1998) page 116