He glanced one more time at the Firm. The office was deliberately forbidding: a mass of turrets and spikes, with a huge inner courtyard. There were a dozen armed and uniformed police officers at the entrance, and no doubt twice as many plain-clothes men keeping an eye on it as well. There was no building in London more heavily guarded. Don’t worry about it, Porter told himself. You can do this. It’s just a matter of holding your nerve.

He walked steadily towards the building, at an even, unvarying pace. Run into this place and they were likely to gun you down. Looking across the river to the clock on Big Ben, he noted that it was just after seven in the morning. Early. But the spooks never slept. And certainly not when they were in the middle of a crisis.

The policemen looked at him suspiciously as he walked across the driveway, and into the entrance lobby, but they made no attempt to stop him. It’s a public building, Porter reflected. They have to at least let people walk into the place. It was only once you checked in at the desks that they ran you over for guns and knives.

The foyer was decorated in grey, with slate floors, and a bank of glass at the front. Marble-clad walls led up to a bank of elevators. Along one side there was a reception desk with three uniformed guards sitting at it. At the other, there was a pair of black leather sofas, and a collection of the day’s papers spread out on a coffee table. All of them, Porter noticed, had pictures of Katie Dartmouth on them. ‘BRING OUR BOYS HOME TODAY’ said huge letters across the front page of the Independent. ‘SAVE HER’ said the Mirror.

Porter walked steadily up to the desk. A thickset man in his forties looked straight at him, his expression as welcoming as a chunk of granite.

‘I know how to get Katie Dartmouth out,’ said Porter.

Even as he delivered the sentence, Porter knew how ridiculous he sounded. If it wasn’t for the fact he knew it was true, he’d be laughing at himself.

The receptionist looked at him, taking half a second to make a judgement, and then reached underneath his desk to press a button. Even though he was trained to remain impassive, Porter could tell he had already reached his own verdict. He thinks I’m a nutter. Who know, maybe he’s right?

Five armed officers appeared from the door that led away from the elevators. They walked quickly across the slate floor. Not running, but moving with purpose. A few people were coming through the entrance doors, heading towards the lifts to get to their desks, but they simply made way for the armed men, paying them no attention. ‘We’ll have to ask you to leave, sir,’ said the first officer, standing in front of Porter.

He glanced at the man’s face. It was expressionless, like rock. He was wearing no uniform but he didn’t need to. There was an MP-5 assault rifle strapped to his chest, and that gave him all the authority he needed.