‘How many miles of river are we looking at?’ I asked.

He shook his head. ‘The Tyburn winds and turns . . .’

‘Ballpark figure, Professor.’

‘It could be as many as ten miles.’

I shook my head.

‘Only someone much more important than me can authorise a search of that scale.’

I called the Chief Super’s office. They put me straight through and I told her what I wanted.

‘Where’s Pat Whitestone?’ DCS Swire said.

The truth is I didn’t know where DCI Whitestone was or if she was ever coming to work again.

‘Ma’am, I believe she must be with her son at the hospital.’

A pause.

‘Do it,’ she said. ‘Send everyone you can down there. But I want them all out at the end of the shift.’

‘Ma’am?’

‘London sewers have the highest concentration of cocaine of any waters in Europe.’

For a moment I had the image of London’s paranoid coke users all flushing away their drug of choice.

But that wasn’t quite it.

‘The city has the highest number of cocaine users in the northern hemisphere and their urine all ends up in the sewers,’ DCS Swire told me. ‘The trace cocaine in London’s waste waters is 500 per cent higher than anywhere else in Europe. If anyone stays down there too long, we’re going to start getting cardiac arrests. And then we’re going to start getting lawsuits. So one full shift and they’re all out, understood, DC Wolfe?’

‘Yes, ma’am.’

I’ll tell them not to inhale, I thought, heading for the door as I speed-dialled Edie Wren.

Tara Jones called me back.

‘I got the voiceprint of that sound we heard on the latest film,’ she said. ‘It’s not a name. And it’s not a word.’

‘What is it?’

‘It’s laughter.’ She shook her head as if she could not understand such a thing, and a veil of glossy black hair swung in front of her lovely face. I watched her push it away. ‘The noise is a short bark of someone . . . laughing. What does it mean?’

‘They’re starting to enjoy it,’ I said.

We had been looking in the wrong place. They were never coming back to the site of Tyburn gallows. So almost one hundred officers – Specialist Search Teams from West End Central and New Scotland Yard, surveillance officers from SO15’s Counter Terrorism Command – spent eight hours of a long summer day wading through the miles of sewers that trace the flow of the Tyburn.

And at the end of a long shift we knew this was the wrong place too.