Mavis Batey, who has died aged 92, was often described as one of the top female codebreakers at Bletchley Park but, while she was always too modest to make the point herself, this diminished her role. She was one of the leading codebreakers of either sex, breaking the Enigma ciphers that led to the Royal Navy’s victory over Italy at Matapan in 1941 and, crucially, to the success of the D-day landings in 1944.
She was 19 years old when she was sent to Bletchley, the codebreaking centre in Buckinghamshire, in early 1940 and put to work in No 3 Cottage, in the research section, which broke into new cipher systems that had never been broken before. It was run by the veteran codebreaker and Greek scholar Dilly Knox, who had not only broken the Zimmermann Telegram, which brought the US into the first world war, but had also pieced together the mimes of the Greek playwright Herodas from papyri fragments found in an Egyptian cave.
In March 1941, Mavis broke a series of messages enciphered on the Italian navy’s Enigma machine that revealed the full details of plans to ambush a Royal Navy supply convoy ferrying supplies from Egypt to Greece. The plans gave Admiral Andrew Cunningham, commander-in-chief of the Royal Navy’s Mediterranean Fleet, the opportunity to turn the tables on the Italians, who were taken completely by surprise. Cunningham’s ships sank three heavy cruisers and two estroyers with the loss of 3,000 Italian sailors. The Italian fleet never confronted the Royal Navy again.
Cunningham visited the cottage to thank Knox and his team of young female codebreakers. “The cottage wall had just been whitewashed,” Mavis recalled. “Someone enticed the admiral to lean against it so he got whitewash on his lovely dark blue uniform. We tried not to giggle when he left.”
In a poem composed to celebrate the victory, Knox dedicated one stanza to Mavis: “When Cunningham won at Matapan, By the grace of God and Mavis, Nigro simillima cygno est, praise Heaven, A very rara avis.” (“Like the black swan, she is, praise heaven, a very rare bird”.) It was, she later said, “very heady stuff for a 19-year-old”.
Arguably, her most important break was into the German secret service Abwehr Enigma. The British had captured most of the enemy spies sent to Britain and were using them to feed false information to the Germans in an operation known as the Double Cross System. But they had no way of knowing whether the Germans believed the false information, because the Abwehr Enigma was deemed impossible to break. Knox and his team believed otherwise and in December 1941, Mavis broke a message on the link between Belgrade and Berlin that allowed them to work out the wiring of the machine.
From that point onwards, MI5 knew the Germans believed everything the double agents told them, allowing them to provide a stream of pieces of intelligence suggesting that the allies had an entire army ready to storm the Pas de Calais.
As the allies landed in Normandy on 6 June 1944, one of the double agents insisted that the main thrust would come against Calais. His report went straight to Adolf Hitler, who ordered two key armoured divisions back to the area. Without the breaking of the Abwehr Enigma, the D-day deception could never have gone ahead and those divisions might well have helped the Germans throw the allied forces back into the sea.
She was born Mavis Lever, in Dulwich, south London, the daughter of a postal worker and a seamstress. She attended Coloma convent girls’ school in Croydon, and was reading German at University College London when war broke out.
While the image of eccentric wartime codebreakers is often exaggerated, it is entirely appropriate in the case of Knox, whose unusual views on training in effect left new recruits to sink or swim. In a chapter on her work in the book The Bletchley Park Codebreakers (2011), Mavis described her own arrival in the cottage where Knox and his team worked, and his first words to her.
“They were: ‘Hello, we’re breaking machines. Have you got a pencil? Here, have a go.’ I was then handed a pile of utter gibberish, made worse by Dilly’s scrawls all over it. ‘But I’m afraid it’s all Greek to me,’ I said, at which he burst into delighted laughter and replied, ‘I wish it were.'”
Mavis met her husband, Keith Batey, a mathematician and himself one of the leading break-in experts at Bletchley, when he assisted her on one night shift to tackle a particularly difficult codebreaking problem. They married in 1942. After the war, she stopped work to bring up a young family.
In the 1960s, when her husband was appointed secretary of the chest, the chief financial officer of Oxford University, they lived on the university’s Nuneham Park estate where the gardens, landscaped in the 18th century, had become overgrown.
Researching the estate, a process that made good use of research skills and a determination to find the truth developed at Bletchley, led Mavis to an interest in historical gardens. She became an immensely inspirational force behind moves by the Garden History Society, the Campaign to Protect Rural England and English Heritage to protect them.
She worked with the Historic Buildings Council to compile what became the English Heritage Register of Parks and Gardens of Special Historic Interest in England and was honorary secretary of the Garden History Society from 1971 until 1985, then its honorary president.
Mavis wrote numerous books on historical gardens, including Jane Austen and the English Landscape (1996) and Alexander Pope: Poetry and Landscape (1999), and a wonderfully affectionate biography of Knox, Dilly: The Man Who Broke Enigmas (2010).
She was awarded the Veitch memorial medal of the Royal Horticultural Society in 1985 and two years later was appointed MBE for services to the preservation and conservation of historic gardens.
Keith died in 2010. Mavis is survived by her three children, Elizabeth, Christopher and Deborah.
• Mavis Lilian Batey, codebreaker, garden historian and author, born 5 May 1921; died 12 November 2013