By Stephanie Holmes

BLETCHLEY (Reuters) - It was the world's most famous wartime cypher -- "Enigma" -- the system of coded messages that held the key to the movement of German forces in World War Two.

It was cracked at Bletchley Park, a windswept mansion in the Midlands where a reunion and festival was held on Sunday to celebrate the achievements of the codebreakers before the release of a film telling their story.

A motley group of crossword addicts, mathematics whiz-kids, 20-year-old debutantes and Oxbridge dons (lecturers) were sent to Bletchley, or Station X as it was code-named, to decipher the communication codes and secret signals used by the enemy armies.

The story of the 10,000 or so codebreakers who worked at Bletchley Park, forbidden to talk to outsiders about their work, has been made into a film called "Enigma" to be released later this month.

The screenplay written by Tom Stoppard is based on a novel by Robert Harris and stars Kate Winslet as a bookish young codebreaker.

From the outbreak of the war in 1939, bright young minds were swept out of the best universities and told they were to be part of the war effort. They were recruited from modern language departments and by way of crossword puzzle competitions planted by the Foreign Office in The Times.

Even after they were told they were to be sent to Bletchley, they never knew exactly what awaited them.

"A letter arrived from the Foreign Office in London and I went down to interview," 78-year-old former codebreaker Sheila Lawn told Reuters at the reunion on Sunday.

"They told me to get off at Bletchley and I was to call a telephone number and the voice at the other end said "Ah, we've been expecting you, Miss Kingsley," she said, quivering at the memory.

VOWS OF SILENCE

Working under the Official Secrets Act, many codebreakers never told their own parents or children what they did during the war and kept their vow of silence for years after the conflict had ended.

"They were all engaged in a project, even though they didn't know the full extent of it, to which they were fantastically committed and I think that draws people together," said Christine Large, Chair of the Bletchley Park Trust fund.

Sheila met her husband Oliver at Bletchley and despite dancing together, falling in love and getting married, neither told the other exactly what they did in their respective huts -- workshops set up in the grounds to maintain an intellectual and physical separation between the different pieces of the jigsaw puzzle of code-breaking.

"You never knew what anyone else did and we didn't talk about it because you couldn't. It was a closed book. For more than 35 years we kept total silence," smiles her 82-year-old husband Oliver Lawn.

He was one of the first to arrive at Station X, summoned from Jesus College, Cambridge University in July 1940 by his mathematics tutor and based in hut six as one of the elite cryptographers working on the German army enigma code.

FIRST COMPUTERS

"The menus that determined how the machines were plugged up were really the first ever computer programmes," he said, blue eyes twinkling.

Simon Singh, an expert on codes and author of "Fermat's Last Theorem" and "The Code Book", agrees that Bletchley Park was the source of one of the earliest computers.

"The whole story of cryptography is an evolutionary one," he told Reuters. "The code-reading machine 'Colossus' was actually a primitive form of computer."

But many of the bright-eyed wartime codebreakers wandering around the grounds on Sunday said the atmosphere was not always overpoweringly intellectual.

"It wasn't just boffins, you know, most of us had come straight from school, I was just 20," smiled 78-year-old Monica Leedham, who worked in the Japanese Naval section -- intercepting codes from Japanese ships from December 1943 until the end of the war. "We were the babes!"

Source: Reuters