A few hours later I stood alone in MIR-1 looking at the floor-to-ceiling map of London and sipping a triple espresso from Bar Italia.

Professor Hitchens came in with his motorbike helmet under his arm, already sweating inside his corduroy.

‘Tyburn,’ he said. ‘It’s a river, isn’t it? That’s where everything else comes from. Tyburn Road, Tyburn gallows – it’s all named after the River Tyburn. The Tyburn is one of the great underground rivers of London.’

‘The gallows is named after the river?’ I said.

He nodded his egg-shaped head. ‘Look at this,’ he said.

He produced a battered book from his saddlebag. Thames: Sacred River by Peter Ackroyd. Professor Hitchens found the page he wanted and pointed a fat finger at a passage. He began to read:

‘There is some intimate association between the river and what we call “paganism”. Something has settled there. The river in some sense becomes the sacred witness of punishment . . .’

He looked at me with his eyes shining.

‘Don’t you see? The river in some sense becomes the sacred witness of punishment!’

Tara Jones walked in and stared at us. Hitchens continued reading.

‘It is perhaps not coincidental that the two major sites of execution on land, Tyburn and Smithfield, were adjacent to the Thames tributaries of the Tyburn and the Fleet.’ He shook his head with wonder. ‘Can’t you see what it means?’

‘Wait a minute,’ I said. ‘Let me get this straight. London’s underground rivers – the Tyburn and the rest – they once flowed over ground?’

‘Yes!’

‘So what happened to them?’

‘We built this city on top of them.’ He waved at the giant map on the wall. ‘As the city has grown over the centuries, the rivers became deeper. The London sewer system is built on the template of the city’s underground rivers. But they’re still there.’

I looked at the map, and back at him.

‘So the Tyburn – the River Tyburn – still exists?’ I said.

‘Of course!’

‘Where does it flow?’ I said. ‘Show me.’

He pointed at a great swathe of green towards the top of the map.

‘The source of the Tyburn is Hampstead. It runs south – parallel to the Finchley Road, down to Swiss Cottage, through Regent’s Park. In the West End it follows the path of Marylebone Lane before passing through Mayfair and into the Thames.’

‘We’re probably standing on it,’ Tara said.

Hitchens’ prematurely aged face split into a wide grin.

‘Savile Row? I would say that it’s extremely likely the Tyburn is directly below us.’

I thought about it, let it settle.

‘They’re not going to go back to the site of the gallows because they know we’ll be waiting. But – if they are so obsessed with the ritual of punishment – they could still leave the body in the Tyburn – the River Tyburn.’

‘It has to be a possibility,’ Hitchens said.