Olympia pursed her lips. ‘Well, Lindsey’s boyfriend Steve is eighteen,’ she said. ‘Does that help?’ To be honest I was too knackered to banter. I checked whether they’d seen strange white guys in hoodies prowling around the tunnels but the sisters said they hadn’t. So I asked if they had somewhere we could wash up, and a working landline.

Chelsea laughed. ‘Landline,’ she said. ‘We have wifi down here.’

They also had a full-on locker room and shower last fitted out, judging by the brass taps and stainless steel fittings, sometime in the 1960s. I guessed it must be a leftover from Kumar’s secret government agency. The girls even managed to dig out a sweat shirt and tracksuit bottoms for Reynolds, who glared at me and Kumar until we remembered our manners and left. We found ourselves waiting in a storeroom filled with bottled water and catering boxes of fun-sized chocolate bars. We washed our faces with the water and had an argument about Mars Bars versus Milky Way and then more water after the taste test. When I judged that Kumar was all sugared up I asked him the difficult question.

‘Is it a total coincidence that you were assigned to this case?’

‘Meaning what?’ asked Kumar.

‘I magic up some lights and introduce you to a pair of river goddesses—’

‘Teenaged river goddesses,’ said Kumar. ‘And it’s not like either of them has done anything particularly religious.’

‘What about the lights?’ I asked.

‘Was that magic?’ he asked.

I hesitated. ‘Yes,’ I said.

‘Really magic?’

‘Yes.’

‘Fuck me!’

‘Now you’re reacting?’

‘Well I didn’t want to embarrass myself in front of the American,’ said Kumar.

‘So you’re not from the BTP version of the Folly?’ I asked.

Kumar laughed and said that British Transport Police had plenty of other demands on its budget.

‘But there is a certain amount of weird shit that goes on down here and people got into the habit of asking me to keep track of it,’ said Kumar.

‘Why was that?’

‘Watched too much X-Files growing up,’ he said. ‘Also I’m a bit of an urban explorer.’

‘So, not your first time in the sewers,’ I said. Urban explorers liked to climb into the secret and abandoned nooks and crannies of the city. That a lot of this involved illegal trespass merely added to the attraction.

‘It’s the first time I ever went surfing in one,’ he said. ‘I come from a family of engineers so I like poking my nose in and seeing how things work. I kept volunteering to do the weird stuff and in the end it became semi-official.’

And thus another arrangement was born.

‘If you ever meet Lady Ty,’ I said. ‘Don’t tell her. That sort of things drives her berserk.’

‘Speaking of X-Files,’ said Kumar, gesturing back towards the locker room. ‘Do you think Agent Reynolds—?’