‘Subterranean London is endless, Max,’ he said. ‘It’s not another city – it’s a thousand cities. Ten thousand cities. What exactly are those search teams looking for? A passage that hasn’t been used for a hundred years? Do you know how many disused tube stations there are in London, Max? South Kentish Town – closed in 1924. Lords – closed in 1939. North End – abandoned in 1906. There are twenty-three abandoned tube stations and God knows how many miles of abandoned track.’

‘Yes, but most of those derelict tube stations are much further out than the British Museum stop.’ I tapped the map on Charterhouse Street. ‘This is the heart of the city. Tara, didn’t you say there was some serious building going on nearby?’

She keyed up the film the Hanging Club had uploaded the night that Abu Din did a runner. The figures were in black shadows. But as the camera slowly tracked across the photographs on the wall of the dead soldiers, the happy, proud and young faces of the Sangin Six, she cranked up the volume of the background noise.

‘It’s not traffic, is it?’

‘No,’ Tara said, studying a line that jolted and jarred across the graph on her laptop. ‘Because the noise sometimes stops. And the traffic never stops. I still think it’s some kind of major building work going on next to the kill site.’

‘But the whole of London is a building site,’ Edie said.

‘Not buildings like this,’ Tara insisted. ‘This isn’t a house in Hampstead or Chelsea being done up for a Russian oligarch. This sounds like a major development – maybe pile columns for a skyscraper foundation. That narrows it down, doesn’t it?’

I smiled at her. ‘It certainly does.’

She looked away, biting her lower lip as we listened in silence to the background noise and she studied her graph. I have no idea how Tara registered the sound but to me it sounded like the gods were doing a spot of DIY.

‘Is that a thousand tons of concrete and steel being poured underground?’ Whitestone said. ‘It could be.’

When the film had ended I turned to Professor Hitchens.

‘There’s something I saw that never appeared on any of their films,’ I said. ‘To get to the kill site, they took me down a corridor that got smaller. It was tiled in the same way as the kill site. White tiles that had rotted with time. White tiles that were so old you could not really call them white any more. They were green as much as white. They were in the kill room and they were in this corridor that got smaller with every step that I took – the ceiling got closer to my head and the walls came in. It was like somewhere in a bad dream But it was real. Did you ever hear of somewhere like that?’

Hitchens shook his head.