Perhaps before the war, debutantes were never asked to do anything serious. When you land yourself in a place like that, it's pretty overpowering. There were people from all walks of life. There were Wrens, there were girls like me, people in uniform, army, navy, air force and later on, of course, Americans. All classes were represented. Especially among the Wrens....

I presume that they must have done a little bit of work on one's background. Make sure that you weren't a Nazi. Because there were a lot of young girls at that time who were mad about going to Germany and thought that Hitler was really rather wonderful. Silly girls. I think they probably wanted to know that we weren't like that...

In terms of class tension, there was absolutely no trouble about that whatsoever. I'd been there about a year and a half at least when it got out that I was an Honourable. And I was frightfully embarrassed about this. Somebody came up to me and said: "Sarah! You're an Honourable!" I said, "No, I'm not really, I'm very dishonourable."

We thought a lot about food. Night watches were especially vulnerable to rumbling tummies and usually forced us to go down to the canteen at 3 a.m., where the food was indescribably awful. It is a well-known fact that to cater for so many people is difficult, and particularly in wartime ... but our canteen outshone any sleazy restaurant in producing sludge and the smell of watery cabbage and stale fat regularly afflicted the nostrils to the point of nausea.

One night I found a cooked cockroach nestling in my meat, if you can dignify it by that name, the meat not the beetle. I was about to return it to the catering manageress when my friend Osla, who had the appetite of a lioness with cubs, snatched the plate and said: `What a waste - I'll eat it!' How she managed to eat so much - minus the insect - and stay so slim I never knew, because any leftovers on any nearby plate were gobbled up by her in a flash.

By February 1942 the Germans had added a fourth wheel to the machine and changed to a new cipher, Shark. And in March the Germans managed to crack Naval Cipher 3, which they dubbed the Convoy Cipher.

The breach in British intelligence was not discovered for five months, even though the Admiralty's Operational Intelligence Centre had set up a special division to protect the security of British naval ciphers. Its 10 staff were totally reliant on Bletchley Park for their expertise yet Bletchley assigned only two people to the task, one the head of GCCS, Commander Edward Travis, who had little spare time for the role. It took the two codebreakers five months to discover that the Germans had cracked Cipher 3. Even then it was not changed until June 1943.

The delay cost lives. At its worst 80 per cent of the messages were read by the Germans, who sank a staggering 1,100 ships in the Atlantic in 1942, causing 10,000 deaths and forcing 30,000 seamen into lifeboats.

A secret report written for the Admiralty after the war warned that the intelligence failure would have to be rectified "if disaster in a future war was to be avoided". The document, written by Commander Tighe of the Admiralty's Signals Division, was considered so "disturbing" that only three copies were ever made. A digest of the report by RT Barrett, stated: "Minor economies in this detail of code and cipher security not only cost us dearly in men and ships but very nearly lost us the war."

Mavis Batey, a Bletchley worker, said that there could have been a breakdown in communication between the codebreakers and administrators which delayed changing the cipher. "It's so much about personalities," she said. "The people who understood what to do and the people who were putting it over were on different wavelengths. The codebreakers were down-to-earth mathematicians and didn't talk the same language as the administrators, who probably didn't understand what they were saying."

But Sarah Baring, 82, who worked at Bletchley before being transferred to the Admiralty's Operational Intelligence Centre, laid the blame at the door of the Admiralty. "We all admired Commander Travis very much," she said. "I think it would have been out of character for him not to have picked up the telephone and talked to the First Sea Lord, with whom he had direct access.

"I am not defending Bletchley just because I worked there. But in my experience this was very unlikely, whereas the Admiralty was a shambles at first. The admirals really didn't want to believe any information we told them. They found it very, very difficult to accept Ultra because we couldn't tell them where or how we got the information. They didn't care for that very much."

In 1938 she did the Season. She had an 18½in waist and reputedly the best legs in London. Her godfather was Lord Louis Mountbatten, and the Mountbattens gave a dance for her at their home on Park Lane. In an interview in 2000, Sarah Baring insisted that in those days the girls in her circle who danced the night away at the 400 Club or the Café de Paris were paragons of virtue: “Nobody told us anything about the facts of life. We were all ignorant, and if we had known we’d have thought it disgusting. Certainly, I and all my close friends would have considered ourselves defiled if we hadn’t come to marriage as virgins. Even after you had become engaged, it made no difference. Virginity lasted right up until the wedding night.

“My mother had died before I got married, so my aunt, Kitty Brownlow, was supposed to tell me the facts of life. But all she said was: 'Don’t worry too much if it hurts - it gets better.’ I thought sex was just for procreation. At deb dances there were a few girls of whom we’d say 'They do it, you know!’ - though perhaps all they did was cuddle and kiss behind bushes. But even that was definitely disapproved of. I never heard of any pregnancies, and can remember no sex scandals at all. If boys tried to pounce, the word soon got around. They were described as NSIT - Not Safe In Taxis - and girls warned each other to avoid them. ”

After war was declared Sarah Norton worked briefly for Vogue magazine, writing captions for photographs of models for five shillings a week. She also wrote articles for the Baltimore Sun in America before signing up as a telephonist at an Air Raid Precautions Centre. She then helped to build Hurricanes at the Hawker Siddeley factory near Slough: “I hadn’t expected the work there to be quite as hard as it was, but we were fired up with the desire to make aeroplanes.”

At this time she was sharing a cottage with another Hawker Siddeley worker, Osla Benning, a Canadian-born beauty blessed, in Sarah’s words, with “dark hair, alabaster white skin, an exquisite figure and a gentle loving nature”. When Mountbatten suggested that his god-daughter might “find a nice girl” for his nephew, Prince Philip, to squire around London she immediately thought of Osla and introduced them.

After a few months at Hawker Siddeley, both Sarah and Osla received letters summoning them to a Labour Exchange in London, where they were tested in their skills in the German language (Osla had been at finishing school in Austria). They passed, and were ordered to report to “Station X” in Buckinghamshire.

Sarah Norton had never heard of Bletchley Park, and when they arrived they were assigned to Hut 4. “Nobody explained anything,” she recalled. “You were merely told that pieces of paper in German would come through and you had to take out any salient information, put it all on to a filing card with the coordinates, and index it. The information we were dealing with was obviously decrypted. Even then we didn’t know the whole picture. We just did what we were told.”

Sarah Norton worked on the Naval Section index, helping to provide details of the U-boats to Hut 8, run at that time by Alan Turing, of whom she once said: “(He) was immensely shy, especially of girls... I once offered him a cup of tea, (and) he shrank back as if I’d got measles or something. He was wonderful. We were all very proud of him.”The work was gruelling, and Sarah Norton and her colleagues took their pleasures where they could: “One afternoon, we decided to give Jean Campbell-Harris, who later became Baroness Trumpington, a ride in a large laundry basket on wheels that was normally used to move secret files. We launched it down the long corridor where it gathered momentum by the second. To our horror, Jean suddenly disappeared, basket and all, through some double swing doors, crashing to a halt in the men’s lavatories. A serious reprimand was administered and our watches were changed so we were distributed among a more sober group.”

She could not help but be struck by the eccentricities of the place: “There was one cryptographer with red hair and a red beard, and he studied Japanese in the evenings as a relief from his cryptography. But in the winter he wore a blue pixie hood on his beard. A pixie hood’s the thing you put on babies’ heads.”