Sixty-six years ago in Block B of the old Bletchley Park, a discovery was made that saved thousands of lives. A young woman, doing the filing, noticed a lot of coded messages, all concerning fuel deliveries to a small village in northern Germany called Peenemünde.
She didn't think much of them at the time. But she reported the information upwards – and the Allies stumbled on the concealed factory site where the Germans were constructing the V1 and V2 flying bombs. An air strike later and the factory was destroyed. Proof, if ever it was needed, that a well-run filing system can help win a war.
Today moss, weeds and wild flowers have taken over in Block B, a place where the course of history was changed. Nature has reclaimed its ceiling and its floor, and unless something is done soon it will either collapse or be pulled down.
Bletchley Park is one of the world's greatest and most neglected wartime monuments. But large parts of it are being left to rot. The north wall of the main building, a 19th-century country house, is now covered in scaffolding, and part of the roof is missing. The Bletchley Park Trust scraped together £100,000 to repair a quarter of the roof. The rest of the building also needs attention, but the money is not there.
In the grounds nearby, a blue tarpaulin covers a whole wall of Hut 6. The windows are boarded; the white paint is so cracked that you could remove it with you thumbnail. It is so unsightly that it would have been broken up for firewood long ago, were it not for its incredible past.
This was where some of the world's cleverest mathematicians pored over passages of gobbledygook trying to find the formula that would convert it into intelligible German – and save Allied servicemen.
Having worked out the key to the code, they passed it through a connecting hatch to the adjacent, Hut 3. That, too, is now an unsightly morass of peeling paint, rotting planks, and boarded windows.
Everywhere around the site there are the same signs of decay. Even on the brick buildings, constructed to be shrapnel-proof, there is chipped masonry, rotting window ledges, chimneys in an unsafe condition, and guttering needing repairs. Weeds are everywhere.
For Block F, where Colossus, the world's first computer, was set to work in December 1943 reading the private messages between Hitler and the top command, it is already too late. It has been flattened to make way for a housing estate and grass pitch.
Bletchley's history since the war has been chequered to say the least. Originally it was the private property of the director of naval intelligence, Admiral Sir Hugh Sinclair, who liked its handy location half way along the railway line that ran from Oxford to Cambridge.
In utmost secrecy it was taken over by the Government during the war where it became home to Britain's code-breakers in a story that would not be told until nearly 30 years later.
The British had been tipped off by Polish intelligence officers that the Germans were using a encrypting machine called Enigma, the updated version of which was so elaborate that the chances of decoding any of his messages were reckoned to be about 158 million million million to one. The Germans and Japanese never wavered in their conviction that Enigma was invulnerable, and the British took great pains not to disabuse them.
A veteran intelligence officer named Dilly Knox, whose hobby was translating ancient papyrus, realised that its existence meant that decoding was going to have to be an industrial process, rather than an operation conducted by a few experts in a single room. When the war began, he set himself up a cottage in the Bletchley stable yard and set the code- breaking operation in motion.
The breakthrough was achieved by an eccentric genius named Alan Turing. He was one of the greatest mathematicians of the century, who is also much admired for how he came to terms with his homosexuality. In Bletchley, there is now a statue of him, made from a million fragments of slate. If Turing and his team had failed, there was a real chance that Britain would have been starved into submission by the damage that German submarines were inflicting on transatlantic convoys.
Perhaps the greatest single achievement of all began when a team of mathematicians set out to analyse an apparently meaningless set of lines on German ticker tape, and imagine what sort of machine was producing them. They correctly guessed the construction of the Lorenz, which they called "Tunny" – the cipher machine used by German field marshals and central command, including Hitler's own staff. As "Tunny" became more sophisticated, it became impossible for the human brain to make enough calculations to decode it within a reasonable time, so the Bletchley analysts invented the machine they called Colossus, capable of reading 5,000 characters a second. There is now a working model in one of the surviving outbuildings, which the trust wants to convert into a museum of computer technology.
After the war, Bletchley Park was used by the government and British Telecom until 1991, when it was in danger of being demolished. At that point the Bletchley Park Trust was assembled to take over ownership. Since then they have raised more than £5m to develop and restore parts of the site. In the past three years visitor numbers have increased by 40 per cent to more than 50,000. But it is still far from safe.
Alarm about Bletchley's future was raised in May, when the Bill & Melinda Gates Foundation turned down an appeal for money and Bletchley Park's director, Simon Greenish, put out a warning that some buildings might have to be pulled down within two years.
This inspired a petition on the Downing Street website, which has attracted almost 11,000 signatures. Then last month 97 senior academics, mostly professors and heads of department, wrote a letter of protest about the state of much of the park.
They said that "the ravages of age and a lack of investment" have left the historic site under threat.
The signatories called for Bletchley to be made the home of a national museum of computing.
"As a nation we cannot allow this crucial and unique piece of both British and world heritage to be neglected in this way. The future of the site, buildings, resources and equipment at Bletchley Park must be preserved for future generations," they wrote. Mr Greenish believes that if he had £10m to spend on renovation, it could be a heritage site to rival the Imperial War Museum. Part of the reason for the long neglect of Bletchley Park is the rigid secrecy under which it operated, which lasted until the 1970s. Some of its 10,000 employees went to their graves without telling even their wives or husbands that the place existed. Asked what they did in the war, they either dissembled or said nothing. Bletchley's staff were, Sir Winston Churchill said, "my geese that laid the golden eggs – and never cackled."
Now though, those that care about one of the most celebrated sites of Britain's recent history are being forced to cackle – to prevent it fading into the past.