It was Vera Kirby's skill at crosswords that landed her a job at Britain's top secret intelligence headquarters. She had just completed her WRNS induction course when she was asked to stay behind for a mysterious interview.

Now 78 and living in North Yorkshire, she remembers the experience vividly. She says: "I had to see some visiting officers and one of the questions I was asked was whether I was good at crosswords. I was lucky because we used to do The Daily Telegraph crossword in our tea-break at work.

"They must have decided I had what they were looking for because I was taken to a holding camp with some of the other wrens. We still had no idea what kind of work we would be doing. We were told it would require us to sign the Official Secrets Act and that if we wanted to back out, that was the time to do it."

That was the end of 1942. In the new year, Vera was transferred to Bletchley Park in Buckinghamshire. The work that went on in the house has been hailed as one of the greatest intellectual achievements of the 20th Century. Its success in cracking the Nazis' Enigma cipher is thought to have shortened the Second World War by at least two years and saved thousands of lives.

But until the ban imposed under the Official Secrets Act was lifted in 1977, very few people were aware of what went on behind the red brick walls of the mansion known as Station X. Now the story of the codebreakers has been turned into a major film, based on Robert Harris' best-selling novel, Enigma, and starring Kate Winslet and Dougray Scott.

Vera hasn't seen the film yet. She says: "I've read the book and it's a good thriller but life wasn't really like that. I imagine the film will make it seem glamorous as well, but most of us were doing very ordinary jobs."

Vera was 17 in 1939. She had just left school and was living in Middlesex with her family. For the first few years of the war she had a variety of jobs, including stints at an Air Ministry testing station and in a hospital pharmacy. But by the autumn of 1942, she decided she wanted to be more involved in the war effort and joined the WRNS.

She says: "I wanted to be called up. Life for girls was much more restrictive then and, more than anything, I wanted to escape from home and village life."

At Bletchley Park, she found herself in a very different world, one where thousands of lives were at stake and secrecy was of paramount importance. She recalls: "The first time we went to the house, we were assembled and told what it was all about and we had a chance to briefly handle an Enigma machine. They were quite open with us about what we were doing, probably to impress on us the need for secrecy. It clearly worked because we didn't talk about the work with each other and, of course, we didn't tell our families."

If anyone asked what she did, she used to tell them she was stuck in the country with the wrens. Her husband, Ken, knew she had been stationed at Bletchley but he had no idea what she was doing until the secrecy ban was lifted 30 years later.

Bletchley Park had been bought by the Government in 1938 and the first codebreakers arrived in the summer of 1939 - a small team of scholars travelling under the guise of Captain Ridley's Shooting Party. By the time Vera was posted there, up to 8,000 people were working day and night to crack Germany's military and intelligence codes. Some were mathematicians and linguists, others were chess players and crossword fanatics.

Vera was assigned to one of the temporary huts in the grounds where she handled messages relating to land forces in North Africa, Europe and the Pacific.

"To be quite honest, the job itself was dull. The crossword was a bit of a hoax because out intellects weren't called on very much. It was a job a schoolchild could have done," she says.

"The messages had rows of call signs along the top and we had to identify which field of war they came from by matching them up against a list we'd been given. We then marked them with coloured crayons and put them in different baskets where they were collected by the important people."

One of the "important people" was Alan Turing, the brilliant Cambridge mathematician who is regarded as the father of the world's first computer. Vera does not remember meeting him but she did come into contact with some of the others.

She says: "They used to come in and pick up the messages. They would always have a few words of chat but nothing about the work. There were people from all over and from all different backgrounds. It was fascinating."

Vera and the other wrens were billeted in the nearby stately home, Wavendon House, and were taken to Bletchley Park by bus each day. Although many of them were occupied in unskilled, repetitive tasks, it was exhausting work and the huts were cold and cramped. But there were compensations and Vera remembers her social life fondly. She says: "Everyone was thrown together and there was a real sense of camaraderie. We were always going to dances."

Although no one was allowed to discuss their work, she believes everyone was aware of how important it was. "There was one occasion after D-Day when a telegram from Monty was read out to us to say that our troops would have been surrounded if we hadn't broken the message so quickly. It made us realise how vital the work was," she says.

"It was a very interesting time for me, especially as a young woman. I remember I was at home at the same time as my brother who was on leave from the Marines. He and my father were talking about the war and I ventured some sort of comment. They said 'what would you know?' and of course I had to shut up. I couldn't tell them what I was really doing. I was sworn to secrecy."