‘The Regiment hid in plain sight in Ulster,’ Mallet reminded him. ‘We’re simply doing the same thing. And it gives us the chance to keep an eye on those pricks in the City.’

Bowman followed him up the steps. They stopped in front of the intercom panel fixed to the wall. Signs either side of the door warned that security cameras were in operation. Mallet pressed a large button and glanced up at the camera above the door, identifying himself to whoever was watching. Several moments passed. Then the door buzzed, and Mallet ushered Bowman into a sparsely furnished enquiry office. The decor was horribly dated: worn carpet, nicotine-stained walls, panel lighting. Every surface was thickly covered in dust. A fat, bored-looking security guard sat behind the glass partition, staring at a bank of computer screens.

Mallet held up his security pass. The podgy guard glanced briefly at it then waved the two men through and went back to screen-watching. Mallet led Bowman down a bland corridor until they reached a bank of lifts. He punched the call button, the scuffed metal doors scraped open, and they stepped inside. Mallet swiped his card against the reader, pushed another button on the panel and the doors thunked shut again.

The lift creaked and clanked as they descended into the bowels of the building.

‘We’re on the lower basement,’ Mallet explained. ‘The range is on the floor above. The lads from SO19 know not to bother us. If we ever want to use the range, we make a call and it’s booked out.’

‘Who else knows we’re here?’

‘Just the top brass at Vauxhall and Thames House. One or two figures in the Foreign Office. The police chiefs. Nobody else. We’re the best kept secret in London.’

Bowman suddenly understood why the Cell had chosen this place as their headquarters. It was fully secure, anonymous and within easy distance of Whitehall, Scotland Yard, the City. No one would ever bother to find out what was going on inside the building, and if they did, the guards would take care of them. The Cell could go about its business in complete secrecy.

The doors scraped open, and then Mallet guided him down another short corridor. They came to a halt in front of a thick steel door secured with a biometric lock.

‘Access is via fingerprint recognition,’ Mallet said. ‘We’ll get your prints into the system as soon as the briefing is over. You’ll be given a security pass, too.’

He pressed his index finger against the sensor panel. The door unlocked with a cheerful beep. Mallet levered the door handle, took a step inside, then paused in the doorway.

‘This is it. Once you step inside this room, there’s no turning back.’

Bowman eased out a breath. ‘I’m ready,’ he said.

‘Good lad.’