This was in 1939 in the days of will-there-or-won't-there be a war and the locals took a poor view of this mixed group of idlers while their own young men were being called up. "It's all right for you, but some of us have work to do," a chambermaid told one of them huffily.

The real secrets of what actually went on in Bletchley Park have come to light only in relatively recent times, and today the place is open as a museum and multimedia centre. What few visitors realise is that many of the hardware and software techniques used in those multimedia presentations and indeed in their own laptops and mobile phones, had their origin in what went on in Bletchley Park more than seven decades ago.

The fictitious "shooting party" of 1939 had been gathered to advise the government on what codes and ciphers to use and to intercept and decode messages not only of Britain's enemies, but also of her friends. Of particular concern, however, were the codes used by Germany who, from the late 1920s, began to send all her military and diplomatic messages using a cipher generated by a device known as the Enigma machine.

This was similar to a portable typewriter with an extra panel of keys in the lid. The operator entered a message on a normal QWERTY keyboard; each letter was changed by Enigma, so that A might become M the first time it occurred and Y the next time and so on. The message was then transmitted as bundles of five letters. The recipient had a machine which could be set in the same configuration as that used by the sender, reversing the code to the original plain text.

A basic Enigma machine could be set in more than 159 million million million ways and the code it generated was regarded by both the Germans and British as unbreakable. At the height of World War II, the machine settings were changed every day, and there were different settings for different departments: diplomatic, police, army, air force, navy, and even different jurisdictions within each department.

As Michael Smith's book, Station X: The Codebreakers of Bletchley Park, recounts, most of those original codeworkers, female and male, were dons or handpicked undergraduates from some of the Oxbridge colleges. Typical of them was Dilly Knox,

Knox was one of a small group who began work on the Enigma code, grumbling about the new men who were being sent to help him: they were mathematicians for heaven's sake, incapable of doing anything practical. One of the first of these was Alan Turing.

Turing was a genuine 24-carat genius of the type the English seem to produce in profusion. Scruffily dressed and favouring his own company, he was a more than competent long-distance runner with a respectable 2 hours 46 minutes for the marathon in an era when athletics was still amateur. At the age of 24, he developed the mathematical theories which underpin today's computers. He is to computer science what Einstein is to physics.

Knox and Turing and their colleagues were making slow progress with Enigma and it was a Polish mathematician named Marian Rejewski who broke the code. Not only did he break it but he constructed two entire Enigma machines from his knowledge of how they worked. He handed over one of these to the French and the other to the British. When the Nazis invaded Poland, he and his team continued their work in France. After the fall of France in 1942, five of them were captured and interrogated by the SS. They never revealed that the Enigma code had been broken.

After January 1940, when Knox and his group finally broke Enigma, secrecy and security were paramount. The Germans believed their code was unbreakable and if they were to discover that it was no longer secure, they could be expected to introduce variations that would put the British – and the Poles – back to the beginning.

Inevitably the Germans began to realise that they needed to tighten up on their security. Admiral Donitz, commander of the submarine fleet, suspected that either the Allies had outstanding agents or his codes were being broken. He ordered that an extra wheel be added to the Enigma machine, effectively giving a code 26 times more complex than the existing one and throwing the cryptanalysts into a panic.

Fortunately, two German code books were rescued from a sinking submarine by three brave seamen, two of whom lost their lives in the attempt. The books were brought back to the rescue ship by the third member of the group, 16-year old Tommy Brown, who was working as a civilian caterer on the ship. Two years later he lost his life when trying to rescue his sister from a house fire. The film U571

As the war went on, the population of Bletchley Park continued to grow, reaching a peak of just under 9000 in January 1945. By this stage, most of those involved were in one or other of the military services and their work was being passed directly to field commanders. Bill Williams, Montgomery's intelligence officer, said about the Normandy invasion, "few armies ever went to battle better informed of their enemy".

Among those working either at Bletchley or one of its offshoots was RAF officer Brin Newton-John, father of the Australian singer; Roy Jenkins, the future Chancellor of the Exchequer; Peter Benenson, the founder of Amnesty International; and Ian Fleming, whose Bond character owed much to those Fleming met when working as a naval intelligence officer.

In the early days of the war, Turing designed a machine that greatly reduced the tedious work of decoding. As the Germans became more security-conscious, bigger machines were needed and in December 1943, Colossus I went into operation, the world's first programmable electronic computer. A number of faster versions were produced and were in almost constant use in the final months of the war.

The idea of using electronic valves rather than relays was the brainwave of a bright young telephone engineer named Tommy Flowers. After the war, two of his machines were kept, but he was ordered to destroy all the others, as well as the drawings, designs and software algorithms. It is only in relatively recent times that the world has come to know about Colossus; few people have heard of Flowers.

Turing was also a casualty of the end of hostilities. In 1952 he was arrested for homosexuality and lost his security clearance. He was convicted of what was then criminal activity and chose to undergo a program of oestrogen treatment rather than go to prison. In 1954, he committed suicide by taking cyanide.

His mother was his first biographer and it appears from her book that she was not aware of her son's sexuality – she quite innocently mentions, for example, that he liked to be around young boys. He was found in his room with a half-eaten apple beside his bed; his mother assumed, as he obviously intended that she should, that he had died as a result of some poison innocently or otherwise associated with the apple.