YOU don't need to wait for Mick Jagger's film Enigma to be released later this year to experience the fascination of World War II's best kept secret.

Bletchley Park near Milton Keynes, where codebreakers worked throughout the war to decode enemy messages, will be the scene this weekend of a re-enactment of a day in May 1944, in the build-up to the top-secret invasion of Normandy.

Meet the academics, officers, WAAFS, WRENS, posh debutantes and servicemen who worked here around the clock as 150 re-enactors recreate the scene. Their work is said to have shortened the war by two years and saved countless lives.

A visit won't be quite like Mick Jagger's film, of course. Enigma, the first movie produced by his company Jagged Films, is based on the romantic thriller by Robert Harris with lots of love interest featuring Kate Winslet, Saffron Burrows and Jeremy Northam. Jagger himself doesn't appear, though his own Enigma machine has a brief but starring role.

It is one of the few remaining Enigma cypher machines, which he bought at Sotheby's because he thought "it might come in handy some day".

These machines look a bit like an oldfashioned typewriter but were capable of putting a message into a very complex code, which the Nazis thought was impossible to crack.

At Bletchley Park some of the finest brains in Britain gathered to break the Enigma codes, building the Turing Bombe decyphering device and the world's first computer, Colossus, to achieve the near-impossible. The Nazis never discovered that their codes had been busted, and the work of Bletchley Park remained a secret until 1974, even though more than 10,000 people worked here on any one day in three shifts.

Bletchley Park has often been in the news since. It was saved from property speculators as a heritage site in 1999, then shot into the headlines last year when a rare Enigma machine was stolen. And more recently the movie U570 raised controversy by portraying that vital codebreaking documents were found by US, rather than British, troops.

Now Bletchley Park is open for visits on alternate weekends, manned by volunteers. While this weekend will be specially fascinating, with re-enactors in period costume making the scene come alive, the 55-acre site is always an intriguing place to visit.

Start at the mansion for an introductory talk followed by a free guided tour pointing out the key points of interest. Then follow the cryptology trail which illustrates the whole process of decoding a message, from its interception to interpretation.

You can also see exhibits of a wide range of wartime memorabilia, from military uniforms to signals technology. The motor transport display includes many military vehicles, while the toy collection will fascinate children of all ages. And don't miss the NAAFI canteen for refreshments. This is where staff relaxed, though all were careful to keep the secret. As Churchill proudly said of them: "These are the geese that laid the golden egg, and never cackled."

Information line (01908) 640404.