Knox now headed east across the city, weaving another elaborate path that took him over the river twice before ending up in another quiet side street just off Finsbury Circus in the heart of the City. From there, he walked along the edge of the bomb-site-turned-building-site that the new Barbican housing estate was slowly rising from. Then, at the edge of Smithfield Market on the corner of Charterhouse Street and Poultry Avenue, he prised open a door that had been designed to look both permanently shut up and derelict, and walked down five flights of stairs.

Smithfield was one of London’s oldest markets, and had been supplying meat and livestock to the city for centuries. In the 1940s the War Office’s Combined Operations Headquarters had requisitioned a large cold-meat store deep beneath it and used the frigid space to test Pykrete, an experimental super-hard but light-weight compound of ice and sawdust dreamed up by Geoffrey Pyke. Senior members of the army and navy had hoped to use it to create huge floating refuelling stations for Allied aircraft and convoys in the Atlantic. But the war ended before Pyke had been able to prove the viability of his wonder material, and then the Combined Operations Headquarters had been promptly disbanded. No one had thought to officially return ownership of the deep store to Smithfield, and there was no one for the market to ask for it back from, so it was left empty and forgotten, by everyone except MI5.

Every morning the market’s Victorian halls were a chaotic stampede of delivery vans and crates piled high with fresh, bloody carcasses, but by the afternoon they were all scrubbed clean and quiet.

The sound of Knox’s footsteps echoed down the stairwell to the cold store. The only other noise was the occasional low rumble of a Metropolitan or Circle line train in the tube tunnel that ran along the northern side of the market.

Enough electricity had been kept flowing for lights to stay on in the store and the lower half of the staircase. But even with power to the cooling systems shut off years ago, Knox still felt a chill run through him as he got closer to his second incredible reunion with Williams.

Their meeting in Regent’s Park had been brief, a cry for help Knox had instinctively reacted to, but he didn’t know what he was now walking into. Williams might be about to tell him about a terrifying threat to national security he had managed to pull from the tangle of his mind, or Knox might be about to face his long-overdue reckoning for sending his best friend to his death.

The last thing he’d anticipated was to be grabbed as soon as he walked into the cold store and pulled into a hug so hard he couldn’t believe he was being held by the same gaunt man he’d sat next to a few hours before.