It was the cigars, mostly. Amid the clatter of typewriters, blurting telephone receivers and constant bustle of the underground bunkers beneath Westminster – the wreaths of blueish smoke winding their way through the labyrinth of corridors were how staff always knew that Sir Winston Churchill was in attendance.

From these dank basement rooms, formerly used by the Civil Service to store moth-eaten furniture, Churchill and his most senior commanders plotted – and executed – Hitler’s demise. Called simply, the War Rooms, and equipped with the very latest technology the country could develop, for six long years they became the nerve centre of the British war effort.

While ensconced in the bunker Churchill delivered the radio broadcasts that ensured the nation’s spirit was not broken during the Blitz, marshalled the defence of the realm and plotted Britain’s own attacks. In the run up to D Day in 1944 he engaged in secret telephone discussions with the US president Roosevelt in a tiny room which staff were told was the prime minister’s personal toilet.

Unlike Hitler’s own impenetrable Führerbunker – dug deep below the Reich chancellery and where the Nazi leader eventually took his own life – Churchill’s War Rooms were always hiding in plain sight beneath Whitehall.