There was nothing to hint to the casual passer-by that number forty-six did not in fact stop at basement level, but continued down into the ground for another five storeys, and continued along, down the Mall, occupying five more houses.

Inside the entrance hall of number forty-six was an attractive brunette in her mid-thirties. What singled her out from the thousands of receptionists in London that looked not unlike her was a cabinet on her sitting-room wall at home which contained some forty-five cups and medals; most were from Bisley, but one had pride of place: it was an Olympic Silver for Rapid Pistol Fire. By making the most local of movements with the hand she used for pushing her intercom buttons, she could, within one and a half seconds of anyone coming through that front door, have produced a hefty Colt revolver and started firing it.

To her left was a glass display booth featuring an array of extremely large uncut gemstones, including a diamond, sapphire, emerald and ruby. A montage of colour photographs behind the stones depicted a drill-bit grinding into the earth, with the caption Amarillo, 1935, a pair of tongs holding up a ruby ring, with the caption Nairobi, 1955, and an aerial photograph of an area of hilly desert.

The display was for the benefit of visitors, not that this building had many strangers visit it, nor did it encourage them. The regular inhabitants of the building knew that the gemstones were nothing but bits of glass, and the photographs were prints from a handful of negatives found in the vaults of the Science Museum.

The entire complex onto which the front door of number forty-six opened housed a daytime staff of seven hundred and fifty people, the majority of whom entered and left via a battery of innocent-looking doorways dotted around the Mall, Trafalgar Square and Cockspur Street, or by one of the three entrances to the car park which sprawled below the complex.

On the fifth floor, through a secretary’s anteroom, and through a massive, heavily beaded double door, was the man who presided over this domain. He was a powerfully built man with a bullet-shaped head rising from a bull-neck; a nose that was long, but did not protrude much, and was broken in a couple of places like a prize fighter’s; his hair was a mixture of dark greys, with occasional black strands, and there were elegant silver streaks either side of his temples. He wore a Turnbull and Asser lawn-cotton shirt, pin-striped down the chest, but with plain white collar and cuffs, a very up-to-date Lanvin tie, and the entire package elegantly wrapped in five hundred pounds’ worth of Dormeuil navy-blue chalk-striped cloth and Hawes and Curtis expertise. He wasn’t a particularly tall man, being no more than five foot nine, but he had the sort of presence that would make the most belligerently drunk of rugger-buggers move over to make room for him at crowded pub bars.