‘Don’t be ridiculous,’ she said. ‘There was the forensics work you people did on his boots. That showed he’d been walking in the sewers.’

‘It’s a big system,’ said Kumar.

‘That it is,’ said Reynolds, who was obviously enjoying herself now. ‘But I did a survey of the manhole covers in the vicinity of the victim’s house and what do you know, one of them was much looser than the others. Had fresh marks around the edge – I suspect from where someone had used a pry bar on it.’

‘You were looking to break Zachary’s alibi, weren’t you?’ I said. ‘See if he sneaked past the cameras using the sewers.’

‘Amongst other things,’ said Reynolds. ‘How far down do you think this is going to go?’

‘If it descends to the same level as the Central Line,’ said Kumar. ‘It could be as deep as thirty metres.’

‘That’s a hundred feet,’ I said.

‘This may come as a surprise to you Constable Grant, but I am conversant with the metric system,’ said Reynolds.

‘Can you hear that?’ asked Kumar.

We stopped and listened. Just on the cusp of hearing I detected a rhythmic pounding, more a vibration in the concrete than a sound.

‘Drums,’ I said and then because I couldn’t resist it. ‘Drums in the deep.’

‘Drum and Bass in the deep,’ said Kumar.

‘Someone’s having a party,’ I said.

‘In that case,’ said Reynolds, ‘I’m so glad I dressed for it.’

The base of the stairway would have been familiar to anyone who’s ever had to schlep down the stairs at Hampstead, or any other deep-level tube station. At the bottom was a grey-painted steel blast door that, much to our relief, creaked open when me and Kumar put our shoulders to it.

We stepped into what I at first thought was an empty tube tunnel, but which I realised a moment later was too big for that – twice the diameter, about the same as a standard platform tunnel. The concrete forms which lined the walls were free of the usual tile cladding, but there was a flat cement floor that was shiny.

I know where we are,’ said Kumar. ‘This is the deep-level air-raid shelter at Holland Park.’

‘How do you know that?’ I asked.

‘Because it’s a deep-level shelter and the nearest one is Holland Park,’ he said.