The Battle of Britain bunker: 75 years on in the RAF control room that foiled Hitler

By the standards of what was to follow, 10 July 1940 was a quiet day in the operations bunker of the RAF's 11 Fighter Group. By dusk, the occupants of the room 60ft beneath suburban London from which Britain's principal line of defence against Nazi invasion was being co-ordinated had overseen the loss of 13 Luftwaffe aircraft at a cost of seven Spitfires and Hurricanes.

Via a bamboozling network combining the cutting-edge technology radar stations and observers armed with nothing more advanced than binoculars, those inside the bunker quietly and efficiently sent four squadrons up into the air to thwart an attack on a British shipping convoy traversing the English Channel.

History only later christened this summer's day the official start of the Battle of Britain - the point in the Second World War at which the Third Reich calculated it could knock a teetering United Kingdom out of the conflict by judiciously applying its overwhelming air power to crush the Royal Air Force. Against the Luftwaffe's 3,000 aircraft, Britain at points could muster just 650 fighters.

Skip forward 75 years and it was difficult to imagine that in the midst of a sprinkling of dilapidated Ministry of Defence buildings hidden in a new build estate on the north western edge of London lies still hidden the top secret nerve centre which helped prove Berlin's calculation wrong by inflicting such swingeing losses on the Luftwaffe that Hitler was persuaded Operation Sea Lion - the invasion of Britain - was no longer worth the pain.

In the bowels of the earth below Uxbridge, the men and women of the RAF beavered for 114 days at the apex of the world's first integrated air defence system to repel waves of Dornier and Heinkel bombers with their Messerschmitt escorts and in so doing lay the foundations for one of the wartime stories that modern Britain rarely tires of telling itself - the country that stood alone against the Nazi machine and staved off defeat with the dazzling feats of the Few.

The importance of preserving what has inevitably become known as the "Battle of Britain Bunker" was highlighted this week when, with the apparently irresistible advantage of also allowing him to take a swing at his Tory leadership rival Boris Johnson, the Chancellor George Osborne announced £1m funding for the restoration of the subterranean redoubt from which Spitfires and Hurricanes were dispatched into combat from across south east England.

In a Budget speech low on gags, Mr Osborne used the announcement to troll the London Mayor for his opposition to the expansion of nearby Heathrow by saying it was time to save the bunker for the nation. The Chancellor said: "Let its renovation stand as a monument of the Battle of Britain and the days when aeroplanes flew freely over the skies of west London."

But as a flight of Hurricanes and Spitfires overflew a skyward-looking monarch to mark the passing of 75 years since the start of the Battle of Britain, the current custodians of the Number 11 (F) Group Operations Room - to give the bunker its official title - were in no doubt of its enduring importance above and beyond serving as a vehicle for Westminster point scoring.

For the last 35 years, the bunker, which is still formally owned by the RAF, has been more or less under the care of Chris Wren, a former RAF Warrant Officer who acts as its curator. With other volunteers, he has kept the building open with a modest income derived solely from visitor donations.

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Mr Wren told The Independent: "It's an incredibly important building. The people who worked here bore the brunt of the Battle of Britain. The decisions made here ensured our safety as a nation. Our future is built upon our past and passing an understanding of that past to the coming generations. This is the room that saved Britain in the 1940s."

Even today, when it is no longer protected by machine guns in pill boxes and the highest level of wartime secrecy that the British state could muster, the bunker requires some determination to find. Around it has sprouted an estate of high-end executive homes and the only indication of a history-changing monument in the midst of so much gunmetal glazing and cedar cladding are two laminated plastic signs to the bunker, one of which points cunningly in the wrong direction.

Once inside, however, is a sight familiar to millions.

Down several flights of stairs and along corridors painted an authentically 1940s institutional beige lies the dimly-lit double-height arena inside which female WAAF plotters famously pushed wooden blocks showing the strength and direction of airborne RAF squadrons around a vast map of southern England and occupied northern France.

Above the map, in a glassed-off eyrie sits the desk and bank of phones from which Air Vice Marshal Keith Park, occasionally accompanied by Sir Winston Churchill, strategised as Britain sought to secure its survival with terrifyingly narrow margins by sending aircraft with just enough fuel into the air to meet a numerically-superior foe whose location had been calculated to the likely nearest mile.

Revisionist historians now question the place of the Battle of Britain in securing the UK's survival, pointing out that even had the RAF been defeated the Royal Navy would have crushed any invasion force.

But if proof were needed of the importance of the bunker in the foundation myth of modern Britain, it was while outside its entrance that Churchill first uttered the sentence: "Never in the field of human conflict has so much been owed by so many to so few."

Built in just seven months, the bunker was the first functioning hub in the defence system devised by Hugh Dowding, the head of fighter command, who saw the advantage of funnelling all information about incoming attacks to a network of regional operations rooms, from which commanders could then judiciously dispatch their forces.

What remains today is the faultlessly preserved plotting room and a riotously diverse museum whose exhibits range from an outsize bust of Hitler, seized in 1945 from the Nazi air ministry in Berlin, to the undergarments of WAAF officers fashioned from parachute silk.

With Mr Osborne's pledge, funded by the proceeds of Libor fines, and the possibility of local authority funding, organisers say they will now take the bunker "to the next level" as a visitor attraction. Whether that includes such spoilsport improvements as clear direction signs remains to be seen.