Reynolds screwed up her face, but she was too winded to argue.

Where the sewer turned through a dog-leg its builders had briefly doubled its width. Halfway up the walls a number of moist brick apertures periodically gushed fluid around our feet. Underneath, one in particular was a heap of vile yellowish white stuff.

‘Please tell me that’s not what I think it is,’ said Reynolds weakly.

‘What do you think it is?’ I asked.

‘I think it’s cooking fat,’ said Reynolds.

‘That’s what it is,’ I said. ‘You’re in the famous fat caves of London – a major tourist attraction. Smells a bit like a kebab shop, don’t it?’

‘Since we’ve lost the FBI’s most wanted,’ said Kumar. ‘Do we go forward or back?’

‘Are you sure you saw someone?’ I asked Reynolds.

‘I’m positive,’ she said.

‘Let’s at least see where this goes,’ I said. ‘Because I do not want to have to come back down here later.’

‘Amen to that,’ said Reynolds.

We pushed on, literally, up the sewer pipe which got gradually narrower until I was walking hunched over. I also started to suspect that the water level was rising – although it was hard to tell, what with the changes in pipe size. To be honest I think we kept going out of misplaced machismo, but by the time we reached the junction we were ready for any excuse. One branch carried on straight ahead while a second branch curved off to the right, both equally narrow, cramped and full of shit.

And like the last temptation of Peter Grant there was, on the left, a slot in the wall less than a metre wide that contained stairs going up.

‘Much as I love standing knee-deep in shit,’ said Kumar. ‘It would be a really bad idea to hang around here much longer.’

‘Why’s that?’ I asked.

‘The water level’s rising,’ he said. ‘In fact, as the senior officer here I think I’m going to insist.’ He stared at us, obviously expecting one of us to object.

‘You had us at “the water level is rising”,’ I said.

We squeezed up the narrow staircase into a rectangular landing where a ladder, which I noted was much more modern than the Victorian wall it was attached to, led two metres up to what was presumably the underside of a manhole.

‘Listen,’ said Reynolds. ‘Can you here that?’

There was a drumming sound from the manhole. Rain, I thought, heavy rain. And also the sound of rushing water, faint but distinct, coming from the opposite corner of the landing. I turned my head and my helmet light illuminated a shadowy rectangle in the floor – the top of a vertical shaft.

Kumar took hold of the ladder. ‘Let’s hope it’s not welded shut,’ he said.

I stepped over to the hole in the floor and looked down.