Knox reached the northbound platform of the Northern Line as a train was about to depart. It was one of the new silver deep-level trains. He stretched his arms out, jamming the nearest set of doors open. Several of the carriage’s other occupants tutted loudly as he climbed inside, angry that they’d delayed their journey home by a few seconds. But Knox ignored them. He kept his eyes on the platform, watching the giant reach the platform too late to stop the train pulling away.

Knox had lost him. But he couldn’t risk the chance that the man had guessed his destination and would follow him all the way to Highgate.

He got off at the next station, Strand. From there he could have run the short distance to Trafalgar Square station and the Bakerloo Line, or doubled back on himself and taken the Northern Line under the river to Waterloo. But he needed to make sure he’d slipped his tail, and that would be a lot easier above ground where he wouldn’t be trapped in trains and tunnels.

Both of the station’s recently installed escalators were cordoned off, so Knox took the spiral staircase that climbed a shaft next to the station’s old, original lifts up to the ticket hall. It was quiet. Knox knew that once he made it up to street level he’d be exposed as he crossed Trafalgar Square, but only for a minute.

He made it three steps up the exit that led to the south-east corner of the square before a boot slammed into his chest and sent him flying back down. He skidded several feet across the floor, sliding to a halt as the body the boot belonged to reached the bottom of the stairs. It was another man wearing an unseasonably heavy coat. This one was a few inches shorter than the giant who had followed Knox to Embankment. But he was almost as wide as he was tall and, from Knox’s viewpoint on the ground, it looked like it was all muscle.

Knox scrambled up to his feet. He heard the sound of footsteps approaching behind him. It was giant number one. Somehow they’d known where he was going and how he’d react if he realised he was being shadowed. He tried to back away as the two giants circled him, but there was nowhere for him to go. In the end, he held up his hands and managed to say ‘Gentlemen—’ before they lunged at him.