I just did what I had to do. It wasn't exciting. I got these messages, I translated them, I decoded them, and simply got through them. They got a lot in. From the radio stations and gun emplacements along the Dutch, Belgian and French coasts. And some in the Mediterranean, but I never distinguished that, and didn't ask too many questions. It was sightings, weather reports, sightings of ships, sightings of aircraft, anything peculiar.

Sheila was recruited for Bletchley Park in July 1943 when she was in the middle of a Modern Languages Honours Degree Course (French and German) at Aberdeen University. She had earlier been `reserved' from call-up as a potential teacher, but decided that she must abandon this reservation and join the War effort. She worked in a part of the Naval Section at Bletchley Park, in Block B. Her main work was to decode and then translate 3-letter codes used by German Coastal Batteries and Radar Stations located around the Channel coasts. She continued at Bletchley until the end of the War in September 1945.

Early in his time at Bletchley, Oliver had joined the Scottish Reels Club, run by Hugh Foss, who was Head of the BP Japanese Section ( though, of course, at the time he did not know this). In due course, after she arrived in 1943, Sheila joined this Club. She was first attracted by Oliver's excellent dancing, and then by Oliver himself. The attraction was mutual, and they soon started going out together, as far as their different shift patterns permitted.
After the War, when they both left Bletchley Park, Oliver had to find a job, and he joined the Administrative Home Civil Service in 1946. Sheila, meanwhile, had to complete her interrupted studies. Since there was at that time no scope for travelling abroad (and what use would a Language Degree be without it?), she changed her course to a General Degree at Aberdeen followed by a Social Science Diploma at Birmingham University.

Then, in May 1948, Oliver and Sheila got married. They lived in London. Oliver worked in several Government Departments, including latterly the Department of the Environment. Sheila initially had a Personnel job with London Transport.

They had 2 sons, David and Richard, and now have 4 grandchildren. On retirement in 1978 they moved to Sheffield, and have been involved there with a number of voluntary and charitable activities.

Oliver and Sheila first revisited Bletchley Park in 1996, 50 years after they had left, and they have made many subsequent visits. They have also, since secrecy was lifted, given many talks to groups, and also interviews on TV, Radio and for Newspapers, about their wartime work. They are delighted to see the continuing public interest in Wartime code breaking - "Top Secret" for so long - and to see the large numbers of people now visiting Bletchley Park.

For 250 years it has exercised the minds of theologians, historians and scientists. Charles Darwin was observed pondering its meaning, and Josiah Wedgwood spent many an hour attempting to decipher its cryptic inscription. Some hope it may hold the secret of the whereabouts of the Holy Grail.

Books have been written, documentary films made and poems penned in an attempt to explain it, but the mystery contained in an 18th century monument in the grounds of Lord Lichfield's estate in Staffordshire has eluded interpretation.

In the hope of succeeding where so many others have failed and finally cracking the conundrum, a group of veteran codebreakers from Bletchley Park arrived at Shugborough House near Milford yesterday, armed with proven grey matter and years of experience in deciphering the German enigma codes.

Tucked away within the 900-acre grounds, they found their puzzle: a stone monument built around 1748, containing a carved relief of Nicholas Poussin's Les Bergers d'Arcadie II in reverse. The picture shows a female figure watching as three shepherds gather around a tomb and point at letters within an inscription carved upon it, which read: Et in Arcadia Ego! (And I am in Arcadia too.) Beneath it the letters O.U.O.S.V.A.V.V. are carved, and underneath them a D and an M.

The staff at Bletchley Park, called in to cast an expert eye upon the monument, could not resist the challenge and turned to some of the surviving members of the team who had spent the second world war deciphering codes.

Viewing it for the first time Oliver Lawn, 85, one of the former employees of Bletchley Park, had no doubt that there was a secret to unravel, contained both in the picture and the inscription beneath, and probably based upon missing letters from classical texts.

Mr Lawn, a Cambridge maths graduate who was among the first civilians to be recruited to Bletchley in 1940, deciphered more than 5,000 German codes during the war, using the enigma machine.

He and his colleagues helped to divert German bombers from British cities by breaking the codes that set the radio beams the Nazis used to lead their planes to the target. The successes of the decipherers is thought to have shortened the war by two years.

Their work was so secretive that it was not until recently that Mr Lawn's wife, Sheila, another Bletchley veteran, discovered what his role had been.

But while Mr Lawn normally succeeded in cracking the German wartime codes, he believes the enigma of Shugborough's monument will not be unravelled easily.

"It is totally different in terms of difficulty to what I used to do during the war," he said. "I think you need classical knowledge as well as ingenuity. This is a language rather than a mathematical code.

"Within its genre I would say it's the most challenging I have ever had to tackle. What we need is a bit more intelligence about the family from the documents held at the estate to try and find a key to breaking this. There is always a key, but if this was a code between two people and only they knew it, it could be almost impossible to decipher."

Over the years there have been a number of theories posited about the meaning contained in the Shepherd's Monument. Chief among these is the belief that the connections of the estate's creators, the Anson family, with the grand masters of the closed society of Knights Templar, and the supernatural myths surrounding the estate - where lay lines meet, rivers cross and UFO spotters regularly gather - are evidence that the carving holds the secret to the Holy Grail.

Other solutions are more prosaic. The current Lord Lichfield's great-grandmother believed the letters represented the lines of a poem from Roman mythology about a shepherdess: "Out of your own sweet vale Alicia vanish vanity twixt Deity and man, thou shepherdess the way."

There is always the possibility that the letters mean very little. Richard Kemp, the estate's general manager, said: "They could of course be a family secret, which everyone in the family knows about and which is of little consequence. But it's like Everest, you climb it because it's there. There's a code here, so everyone wants to unravel it."