I showered and changed my clothes at West End Central but I felt that I could still smell the ancient stink of subterranean London on my skin. MIR-1 was deserted apart from Hitchens, who was sitting at a workstation reading his Peter Ackroyd book. I stared up at the great map of London that covers one wall of MIR-1.

‘Where does it come out?’ I said.

‘What?’ He didn’t look up from his book.

‘This river. The River Tyburn.’ I took a step towards the map. ‘The Tyburn is a tributary of the Thames, right?’

Now he was looking up.

‘Yes.’

‘So it doesn’t flow into the sea,’ I said. ‘And it doesn’t disappear underground. At some point the River Tyburn flows into the River Thames.’

‘That’s correct.’

‘Where?’

He quickly pulled his iPad from his saddlebag and found an ancient map of London.

‘The Rocque map of London in 1746,’ he said. And then, ‘Vauxhall Bridge.’

‘So the Tyburn flows into the Thames at Vauxhall Bridge?’

‘Yes.’

‘It will be quicker if we take your bike,’ I said.

Vauxhall Bridge rose up before us as Hitchens tore down Millbank on his old 500cc Royal Enfield with me riding pillion.

Downriver the sun was sinking behind Battersea Power Station. On the far side of the Thames I could see the great tiered building housing MI6, the Secret Intelligence Service, at Vauxhall Cross. Hitchens steered his bike in the empty forecourt outside Tate Britain and we left it there.

We found some stone steps that led down to the Thames Path, the walkway that runs along the riverbank. I started towards the bridge, Hitchens struggling to keep up with me.

‘Down there,’ he panted. ‘A culvert.’

I was directly opposite the MI6 building when I saw it. A large round hole punched into reinforced concrete, big enough for a man to stand up in, pouring a shallow but steady stream of water into the Thames. The culvert was one level lower than the Thames Path, and I realised that it was invisible from the road.

‘Is that it?’ I said. ‘That’s the Tyburn?’

I don’t know what I had been expecting.

His breathless voice was behind me. ‘According to Rocque—’

But I was already going down the steps that led right on to the riverbank and so I missed what Rocque had noted in the eighteenth century. I stepped into the culvert and the water covered my shoes. I took another step and peered into blackness. But the concrete culvert looked too modern to mark the end of a river that had flowed here for thousands of years.

Hitchens hesitated at the mouth of the culvert, keeping his feet dry.

‘This can’t be it,’ I shouted, and my voice echoed back to me.

‘What’s that?’

‘I said—’

And then I saw the body.