A handwritten book containing notes and calculations made by Alan Turing while he worked on decoding Enigma  has been sold at auction for £700,000. The 56-page document, believed to be the only extensive Turing manuscript in existence, was written in 1942 at the Government Code and Cypher School at Bletchley Park, in Buckinghamshire. The notebook, in which he details his work on the foundations of mathematical notation and computer science, was sold for $1,025,000 (£698,000) in a Fine Books & Manuscripts Sale at Bonhams in New York on Monday. Scroll down for video . Turing (left) wrote the notes at the Bletchley Park code-breaking headquarters in 1942 and they paved the way for computer science. The notes are believed to be the only extensive Turing manuscript (right) known to exist, the auctioneer said . A handwritten notebook in which Britain's Enigma machine genius Alan Turing admits he is baffled by an equation could fetch up to $1million (£690,000). Above, he wrote that the Leibniz notation was 'extremely difficult' Turing is best known for his contribution to cracking the code used by the Germans in their Enigma machines during the Second World War while working at at Bletchley Park, in Buckinghamshire. He is often said to be the father of modern computing and artificial intelligence. Benedict Cumberbatch, who played Turing in the Oscar-nominated The Imitation Game, said: 'Alan Turing was a war hero, who broke the Enigma code helping to bring an end to World War II, the father of modern computing science, and a gay icon who lived in a time of intolerance and tragically committed suicide as a result. 'His impact on our everyday lives is enormous, and the thought of holding a manuscript by him is thrilling.' The notebook was among the papers he left in his will to friend and fellow mathematician Robin Gandy. Benedict Cumberbatch, who played Turing in the Oscar-nominated The Imitation Game (left), said: 'The thought of holding a manuscript by him is thrilling' Pages of notes show the inner workings of Turing's thoughts as he helped break codes and decipher tests . The 56-page book bought in Cambridge contains Turing's thoughts on the clearly tricky 'Leibniz notation dx/dy'. 'I find [it] extremely difficult to understand in spite of it having been the one I understood the best once!' he wrote. Gandy gave the papers to The Archive Centre at King's College in Cambridge in 1977. But he kept the notebook, using its blank pages for writing down his dreams at the request of his psychiatrist. Bonham describes Gandy's entries as highly personal; the notebook remained in his possession until he died in 1995. At the beginning of his journal, Gandy writes: 'It seems a suitable disguise to write in between these notes of Alan's on notation, but possibly a little sinister; a dead father figure, some of whose thoughts I most completely inherited.' Prior to the sale, Andrew Hodges, a leading Turing scholar, said: 'Alan Turing was parsimonious with his words and everything from his pen has special value. 'This notebook shines extra light on how, even when he was enmeshed in great world events, he remained committed to free-thinking work in pure mathematics.' The notes, which are highly personal, show Turing working through different equations and considering different results . In some Turing elaborates on his conclusions, writing that one result 'is too subtle and makes an inconvenient definition' But Turing died from cyanide poisoning in 1954, generally thought to have been an act of suicide as a consequence of the hormone treatment he was undergoing as an alternative to imprisonment for his homosexuality. Prime Minister Gordon Brown apologised on behalf of the British Government in 2009 for the way Turing had been treated and he was granted a posthumous pardon in 2013. It will be sold by Bonhams in New York on Monday. It said the seller wished to remain anonymous. Part of the proceeds will be donated to charity. Cassandra Hatton, senior specialist in Fine Books and Manuscripts and the History of Science at Bonhams, said: "This is a wonderful result and a fitting testament to Alan Turing's impact and legacy. 'It has been a great privilege to have been involved in this sale and we are immensely pleased that all the people who bid for this unique item and indeed the wider public have recognised Turing's importance and place in history.' Bonhams said a rare three-rotor German Enigma Enciphering Machine sold at the same auction for a world record price of 269,000 dollars (£183,000). A portion of the proceeds from Bonhams and the vendor will be donated to charity, the auctioneers said.
Notebook written by Alan Turing during World War II sold for £700,000 .
It was written at the Bletchley Park code-breaking headquarters in 1942 .
Only extensive Turing manuscript thought to exist, auctioneer said .