So was I, to be honest, I hadn’t wanted to go down the manhole, but once I’d made myself do it I was all right. It helped that I was surrounded by people I trusted. As Conan the Barbarian famously said, That which does not kill us does not kill us.

‘Where to now?’ I asked Zach.

He gestured down towards what I now knew was the North Kensington Relief sewer, far too low-ceilinged to walk along upright. The CO19 guys, who were understandably thrilled to be heading down a long straight pipe, wanted to wait until they’d fetched up a set of ballistic shields. But Nightingale waved them back.

‘We’ll do a recce first,’ he said and gestured me and Zach to go with him. The CO19 officers gave us pitying looks as we followed Nightingale into the tunnel. Now, I have allergic reaction to getting out in front of armed officers, but Zach didn’t seem bothered. Either he wasn’t expecting trouble or he had more faith in Nightingale than I did.

We made our way down the tunnel for about twenty metres when Zach told us to stop.

‘We’ve gone past it,’ he said. ‘Sorry.’

We backed up two metres while Zach banged his fist at regular intervals on the left side of the tunnel. He stopped suddenly and banged the same spot a few times.

‘This feels like it,’ he said.

I put my hand on the wall where he’d smacked it. There was definitely something like a flash of an open oven and that hint of the pigsty – although given that we were in a sewer that might have been from elsewhere.

Nightingale put his hand next to mine.

‘Extraordinary,’ he said. ‘How do we get in?’

‘Like so,’ said Zach and, turning, put his back against the wall. Then, bracing one foot on the opposite wall, he pushed backwards, forcing a section of the wall to retreat into a recess. The walls were smooth and coated with the same ceramic finish I recognised from the fruit bowl. There was a dull click and the section behind Zach locked into place.

‘Not bad huh?’ he said and pointed upwards. Above him was an open hatch into darkness. ‘It’s like a fire door so it closes automatically. Someone needs to hold it open while I climb up.’

Nightingale lifted his hand and made a small gesture and the movable bit of wall shifted slightly and clicked. Zach gingerly shifted his shoulders.

‘Or you could do that,’ he said.

Nightingale called along the drain for the rest of the party to come up, leaving two of the CO19 officers to guard the junction and two more to man the tunnel. Then he swarmed up through the hatch and, turning, reached down to help me up behind him.