‘Stay safe,’ she said and hung up.

I was just wondering if we were going to end up walking all the way to Notting Hill when Kumar found the stairs. It was a spiral staircase wrapped around a thin cast-iron hub, unmistakably late Victorian – who else would expend that much effort on something no one was ever going to see? It was impossible to tell how far down it went, although I caught a strong whiff of excrement and bleach wafting up from below.

‘That’s the sewers,’ said Kumar. ‘No mistaking that.’

Beyond the entrance to the staircase the passageway continued in its curve to the left.

‘Down the stairs or keep going?’ I asked.

‘We could split up,’ said Kumar with more enthusiasm than I liked.

The floor of the passageway beyond the stairs seemed a paler colour under my head light than the section we’d just walked. I squatted down and had a closer look. There was definitely more dust on the far side and it seemed less disturbed. I admit it wasn’t much, but it was all we had and there was no way I was splitting up.

I explained my reasoning to Kumar, who cracked a glow tube to mark the spot and made a note on his map.

‘Down it is,’ he said.

We went down slowly counting revolutions as we went. Three turns down we encountered a landing with a doorway – the stairs continued downwards. When I had a look through the doorway the shit and bleach smell was strong enough to make me gag. The room beyond was barely larger than a broom cupboard and most of the floor was taken up with an open hatchway. Holding my nose and breathing through my mouth I peered down. Below I recognised one of Bazalgette’s famous sewers, complete with egg-shaped cross-section and sturdy English bond brick lining. It was over a metre across at its widest point and a quarter filled with surprisingly watery-looking water considering what it smelt like.

‘Tell me they didn’t take their food through that,’ I said.

‘Definitely an FSA violation,’ said Kumar. ‘We don’t want to go down there. I’m not qualified for the sewers.’

‘I thought you went caving in wild out-of-the-way places,’ I said. ‘Caves that no man had caved before.’

‘And none of them were as dangerous as the London sewer system,’ he said. ‘Or as smelly.’

I examined the hatch. It looked cast-iron and late Victorian. It also had the same ceramic camouflage as the door in the cut fused into its underside.

‘This is obviously designed to be closed.’ I swung the hatch back and forth a couple of times to demonstrate that it wasn’t rusted open or anything. ‘Somebody left it open, probably because they were in a hurry, and I think we’ve got to check it out.’

‘You know I’ve heard rumours about you,’ said Kumar.

‘Are any of them true?’