By now the Firm had secured plans of the flat that Barrakuda was using. Markham Court was a small red-brick block, dating from the 1930s, in Seymour Place, north of Marble Arch. It belonged to West End Homes, a property company, and in June apartment No. 10 had been taken, fully furnished, on a three-year lease by a firm based in Malta. The area was up-market residential, central and convenient, and in recent years had been heavily infiltrated by Arabs.

The building had only five storeys, and No. 10 was on the top floor. A single lift went up from inside the front door of the building, with a staircase winding round the outside of the shaft. Lift and stairs both gave on to small landings, with two flats on each floor, to right and left. The only other access to each apartment was via a metal fire-escape, which served a back door leading out of the kitchen area.

Only five floors, I thought. The height’s no problem. After our sixteen-floor epic in Moscow, this was money for jam in technical terms. The problem was going to be spectators: once explosions started cracking off, people would inevitably assemble to gawp. Still, that was a matter for the police.

The assault was easily planned. There was no need for anything elaborate like an abseil drop off the roof: all we needed was for our Red and Blue teams to arrive at front and back of the building simultaneously and secure the exits. Red would commandeer the lift and at the same time clear the front stairs. Blue would do the same at the back and go up the fire-escape. With the teams co-ordinated by covert radio, we’d blow both doors and storm the flat – the aim being, in the first instance, to overpower the people inside rather than kill anyone and possibly rob ourselves of vital intelligence. Only if we met armed resistance would we use our weapons, and only when the bomb had been found would we get rid of Barrakuda.

On the floor of the warehouse we laid out white mine tape to the exact dimensions of the rooms in the flat, and decided who would clear which. In the building across the street from the target, one startled family had to be evicted from their penthouse so that sniper observers could be installed and listening equipment brought to bear on the windows. Early indications suggested that the flat was occupied by two men.

Meanwhile, a Russian-speaking policeman – a young, dark guy called Michael, who looked more like a student than a police officer – was seconded to Red team, with orders to come up and join us as soon as the flat was secure. Two nuclear technicians from Porton Down were standing by to neutralise Orange, when or if we found it.