Willoughby-Brown nodded. ‘Let’s do it.’

Mohammed al-Hussain lay on the table, the barrel of the rifle supported by the cushion. He brought his eye to the scope and settled his shoulder against the stock. He was breathing tidally, his chest barely moving. He centred the crosshairs on the chest of the black boxer in the ring and imagined pulling the trigger, then the bullet slamming into the man’s flesh. He smiled at the reaction it would cause, shot in the heart in front of thousands of spectators, with television cameras beaming the pictures around the world.

He moved the gun a fraction, focusing on one of the trainers who was shouting encouragement at his boxer. He centred the crosshairs on the man’s face and imagined it imploding as the bullet hit it, spraying blood and brain matter over the crowds behind him.

He rolled his shoulders, relaxing the muscles that had tensed in anticipation of what was to come, then settled back into the firing position. He scanned the VIP section, which was starting to fill. He recognised some of the faces. A leading Russian industrialist. A newspaper owner. An actor and his model girlfriend. Several famous boxers. The crosshairs passed over them all.

Al-Hussain moved the scope away from the ringside and scanned the terraces. He settled on a middle-aged man selling programmes. He was in his forties, balding and wearing a purple bow tie. He was holding several copies, like an oversized poker hand, and a bag over his shoulder contained dozens more. Al-Hussain slipped his finger over the trigger, took aim, held his breath, and fired.

Shepherd stared up at the floor indicators above the three lifts. One was on the ground floor, the doors open. The middle one was at the top. The final lift was on the sixth floor, coming down. There was no way of telling for sure which floor the jihadists had stopped at. Any hope of accessing the CCTV footage had been dashed when they had gained entrance to the block. There was no one on duty in the reception area, just a notice that gave a contact number for the managing agent.

‘Top floor?’ said Willoughby-Brown, his uncertainty showing in his voice.

‘Who knows?’ said Shepherd. ‘It’d be a guess.’

The three armed officers were looking at them for guidance.

The lift that was on the way down passed the third floor. Hands were creeping towards concealed weapons. The lift passed two and one … Then the doors opened. An old lady reversed out, pulling a wheeled shopping basket as she muttered to herself. They watched her hobble to the doors, fumble for the exit button, and let herself out.

As the door opened they heard a helicopter overhead. Shepherd turned to Willoughby-Brown. ‘The Met’s chopper,’ he said.

Willoughby-Brown understood immediately. The Metropolitan Police’s Air Support Unit had three Eurocopter EC145 helicopters, each equipped with night vision and infrared cameras. He tapped his Bluetooth earpiece. ‘Wendy, patch me through to whoever’s running Met Control over at Bow.’