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Ann Mitchell, Oxford maths graduate recruited as a Bletchley codebreaker – obituary

Ann Mitchell in Edinburgh at the age of 92 - Jane Barlow/Scotsman Publications 
Ann Mitchell in Edinburgh at the age of 92 - Jane Barlow/Scotsman Publications

Ann Mitchell, who has died of Covid-19 in a care home aged 97, worked at the heart of the code-breaking nexus in Hut 6 at Bletchley Park during the Second World War.

The hut was where women with degrees in law, maths and economics converted the “cribs” (pieces of German text that might appear in messages), from cryptanalysts in the next room, into “menus”, or diagrams, to enable Wrens working on Alan Turing’s bombe machine to crack that day’s German army and air force messages. Every night at midnight the German encryption changed and the work started again.

The hut was staffed entirely by women, owing to Civil Service rules that prevented women from working on night shifts with men, to preserve “decency”.

In 1940 Ann Williamson, as she then was, had been one of only five women in her year accepted to read Mathematics at Oxford. After graduating, she started in the autumn of 1943 as a “temporary assistant in the Foreign Office” at Bletchley Park, on a salary of £150 per annum.

“I was terribly pleased when I got the job. I didn’t want to go into the Armed Forces and do drilling and wear a saggy uniform,” she told Tessa Dunlop, author of The Bletchley Girls (2015).

Ann Mitchell tests an Enigma machine unveiled at Edinburgh's Royal Museum, 2001 - David Cheskin/PA
Ann Mitchell tests an Enigma machine unveiled at Edinburgh's Royal Museum, 2001 - David Cheskin/PA

Fortified by a cup of Oxo and a boiled egg, Ann would cycle 10 miles from her lodgings to her night shift every day: “We all sat at long trestle tables working round the clock in eight-hour shifts. We were given a message to decode, and did so with the help of a machine.” Security was so tight that she never even heard the word Enigma or understood the significance of her work.

Yet, she recalled, “I absolutely loved it. The mental challenge was very satisfying, the fact that we didn’t know what it was all about was neither here nor there.

“We would get strings of code, just letters, Morse from the listening station, then next door they decided what it possibly said. We had to make diagrams to link the pairs of letters together.

“What was enormously helpful was a date or one at 6:30am saying ‘nothing to report’. You could begin to work it out and that gave us what we called the crib for that day.

“We never really imagined that with every code we cracked we could be saving thousands of British troops from death.” Like thousands of others who worked at Bletchley Park, for years afterwards Ann kept her wartime work a secret, telling family and friends that she had worked as a secretary in the Foreign Office.

She was shocked when journalists and historians began spilling the beans: “It was the early 1970s, and the first book about Enigma and Bletchley Park came out. I remember shouting: ‘He can’t write about that – we’ve all signed the Official Secrets Act!’ ”

It was then that she told her husband Angus about her role, but it was only with the release in 2001 of the film Enigma, dramatising the world of Bletchley Park, that Ann Mitchell felt she could talk freely about the work of Hut 6.

Indeed, as she told The Scotsman, it was the film and the attendant publicity that enabled her to fit the “bits of the jigsaw” into place in her own mind: “All those decades, and I had no idea what part in the great big machine our little cog was playing. Now everyone is talking about it.”

Ann Katharine Williamson was born in Oxford on November 19 1922, the eldest child of Herbert Williamson, a former commissioner with the Indian Civil Service in Simla, and his wife Winifred (née Kenyon), who helped to run one of Britain’s first family planning clinics.

A scholarship took her to Headington School, Oxford. Her German teacher reported that “she works hard and often with good results in compositions”, while her maths teacher described her as “quick to grasp new ideas”. But her headmistress sought to prevent her from applying to read Maths at university, judging the subject “unladylike”. Luckily her parents disagreed; she graduated from Lady Margaret Hall in 1943.

On VE-Day in 1945, she and her colleagues at Bletchley were told that their services were no longer required. She went on to work as a secretary at an Oxford college and in 1948 married Angus Mitchell, who had been studying History at Oxford after wartime service in which he won an MC. The couple moved to Edinburgh, where he got a job in the Scottish Office and they brought up four children.

Ann became a counsellor for the Marriage Guidance Council in Edinburgh, and began to be increasingly concerned about the effects of divorce on children. She undertook research which led to an MPhil at the University of Edinburgh and prompted changes to Scottish law to ensure that children’s interests are properly taken into account in divorce settlements. She also wrote several books on the subject, notably Children in the Middle (1985).

Ann Mitchell’s husband died in 2018 and she is survived by their two sons and two daughters.

Ann Mitchell, born November 19 1922, died May 11 2020