Last Weekend: Do you want to know a secret? The only way is Bucks

(PA)

Careless talk costs lives, but careful listening can save souls by the millions: that is the message at the Buckinghamshire home of the codebreakers who helped to turn around the Second World War and hasten Nazi Germany's fall.

Last weekend the heavens emptied intermittently on the hundreds of visitors to the warren of old huts clustered around the sub-stately home of Bletchley Park. Huddles of brollies provided an endearing dimension to the picture of how Britain beat Blitzkrieg: with magnificent mathematics.

In the late 1930s, as the spectre of war approached, military chiefs needed somewhere to decode messages intercepted from the enemy. Stipulations for the location included a large telephone exchange nearby, and good rail access. Bletchley Park, a few hundred yards from the West Coast Main Line and a few miles from a new GPO exchange at Fenny Stratford, was just the place.

The added bonus of the now-defunct "Varsity Line", connecting Oxford with Cambridge, provided rapid access for the brainiacs who soldiered unsung through six years of wars. Mathematicians provided the final essential to enable Britain to defeat a stronger enemy: breaking the "Enigma" code.

The Germans adopted a commercial encryption machine for wartime communications. The secret of Enigma was the boggling number of possible settings – roughly the same order as the number of grains of sand in the world – and a sequence that was changed every day. Unbreakable, the Nazis concluded. Their misjudgement was crucial to the course of the war.

Building on work by Polish codebreakers, passed to Britain shortly before war broke out, the brilliant mathematician Alan Turing overcame the odds. Soon, the thousands of staff at Bletchley Park were routinely deciphering and translating top- secret orders. The War Ministry now had the welcome problem of preventing Germany from knowing Enigma had been broken. It was solved by methods such as sending fake messages to non-existent spies thanking them for their excellent work.

Thank goodness the Bletchley Park experience is not (yet) an Experience. Peter Chilvers was the excellent volunteer guide who ushered us through the site and the showers, revealing Britain's make-do-and-mend war effort: in the absence of the internet, MI6 deployed a web of 200,000 racing pigeons; and the codebreakers' mailing address – PO Box 111, St Albans – was a branch of Dorothy Perkins.

Next weekend: the site opens 9.30am-5pm (01908 640404; bletchleypark.org.uk).