Today marks the 70th anniversary of the largest sea-borne invasion in history.

On 6th June 1944 little short of 7,000 vessels were used to transport troops and equipment in a counter offensive against Hitler’s German occupation of Europe.

Should that have failed, and the predictions were far from encouraging, London had a last ditch defence.

[I]f Nazi jack-boots marched up Whitehall the last obstacle they would encounter was a concrete monolith beside St. James’s Park – The Admiralty Citadel. Aesthetics were not the top consideration in its construction. The fort was described by Churchill as: ‘vast monstrosity . . . the demolition of whose twenty foot thick steel and concrete walls will be a problem for future generations.’

Lacking the enthusiasm or technical know-how (short of explosives) with true British pragmatism its stark bleakness has been softened by a covering of ivy.

To disguise this vast structure from the air a grass lawn was laid on the roof so the building merged with the adjacent park. Until recently its greensward was mowed to the astonishment of passers-by on hearing a lawnmower above their heads.

The stronghold has gun emplacements giving a 360-degree field of fire, protecting the bomb-proof bunker beneath the building which was a communications hub for military commanders with direct lines across the globe and to every British ship.

A secret network of tunnels code named Q-Whitehall fan out accommodating several thousand personnel should the need arise.

Half hidden in the shrubbery is the base of a gun emplacement the last of the 4 defensive stop lines to, well, stop advancing tanks, that once encircled London the outermost starting at a point that the M25 now occupies.

Most of the wartime administrative buildings have either been abandoned or are still closed to the public. A rare accessible survivor remains in Dollis Hill.

We will have to wait until Government files are released in the 2020s to discover the extent of these excavations and if its roof garden is still maintained.

Photo: Peter Russell