But how had someone got in? They would have needed a key.

The next morning, Erika tidied the flat and was contemplating calling in to the station that she may have had a possible intruder – possible being a very accurate word – when she heard the post land on the mat downstairs. After sorting through the bills for her neighbours and leaving them on the table by the door, she found a letter addressed to her. Her first piece of mail in her new flat. It was a request from the Met Police that she attend a psychiatric evaluation in seven days’ time.

‘I’m not crazy, am I?’ said Erika to herself, only half joking. When she came back up to the flat, her phone rang.

‘Erika, it’s Marsh. You’ve got six hours with a team from Thames Water. If you don’t find the phone, then that’s it. You understand?’

Hope rose in Erika chest. ‘Yes. Thank you, sir.’

‘There’s virtually no chance it’s down there. Have you seen the rain we’ve been having?’

Erika looked out as the rain hammered against the window.

‘I know sir, but I’ll take those odds; I’ve solved cases on less.’

‘But you won’t be solving this. You’re suspended. Remember? And you’ll pass any evidence over to DCI Sparks. Immediately.’

‘Yes, sir’ said Erika.

‘Moss will be in touch with the rest of the details.’

‘Very good, sir.’

‘And if you ever pull a stunt like that again, showing up on my doorstep and waving sick crime scene photos in my wife’s face . . . You won’t just be suspended. Your career will be over.’

‘It won’t happen again, sir,’ said Erika. There was a click and Marsh hung up. Erika smiled. ‘Behind every powerful man is a woman who knows how to push his buttons. Good on you, Marcie.’

Erika walked over to meet Moss and Peterson. The manhole accessing the storm drain was beside the graveyard at Honor Oak Park Church, only a couple of miles from Erika’s flat. The church was a few hundred yards past the train station, perched on a hill. The rain had stopped, and there was a slight break in the clouds when Erika met Moss by a large van bearing the Thames Water logo. Peterson had a tray of takeaway coffees and was handing them out to a group of guys wearing overalls.

‘This is Mike. His team will be coordinating the search,’ said Moss, introducing them.

‘I’m Erika Foster,’ she said, leaning over to shake hands. The guys didn’t mess about. They gulped down their coffee and within minutes they were levering up the giant manhole cover and rolling it to one side with a clink.

‘Good to see you, boss,’ said Peterson, handing her a coffee with a grin.

Mike took them into the tiny van. It was equipped with a bank of monitors, a small shower, and radio comms for all the men going down into the drain. On one of the monitors, a satellite weather map continually refreshed, showing streaks and bulges of charcoal grey across a map of Greater London.

‘That’s the difference between life and death,’ said Mike, tapping a biro against the screen. ‘The sewers below combine storm water and waste water. A sudden downpour of rain can flood the sewers, and very quickly you have a tidal wave of water making its way towards the Thames.’

‘What did you do before all this technology?’ asked Peterson, pointing at the television screens and satellite weather maps.

‘Good old fashioned noise,’ said Mike. ‘If a storm came, we’d lift one of the nearest man hole covers six inches and let it crash back down. The clanking sound would echo down the tunnels and hopefully give the blokes down there enough of a warning to get the fuck out.’

‘Is it just blokes who work down there?’ asked Moss.

‘Why? You want to apply for a job?’ quipped Mike.

‘Very funny,’ said Moss.

They came back out of the van and looked at the sky. The cloud above seemed to be clearing, but was growing darker on the horizon.

‘We’d best get on with it,’ said Mike, moving over to where the four men had set up a winch above the manhole, and were attaching themselves to safety harnesses. Erika went and peered down the shaft where iron rungs stretched away into blackness.

‘So what are we looking for, a phone?’ asked Mike.

‘It’s an iPhone 5S, we believe it’s white, but it could be black,’ said Moss. She handed them each a laminated photo.

‘We realise it’s been down there for almost two weeks, but if you find it, please can you avoid touching. We need to preserve any remaining forensic evidence. I’ll give you these evidence bags, which it will have to be placed into immediately,’ said Erika.