The superintendent’s overriding concern was always how he came across on screen or in print. He would be doing his bit to camera outside Colindale station later that day. He would dispense the simple, shocking facts, emphasising the brutality and the horror of what had been done to Emily Walker and letting it be known that any steps necessary would be taken to bring her killer to justice.

Thorne had to give the man his due. He couldn’t catch a council-tax dodger if his life depended on it, but he did righteous indignation pretty damn well.

‘It’s someone she knew,’ Thorne said. ‘Someone who’d been watching. She’d seen him around, spoken to him, whatever.’

Brigstocke nodded. ‘Let’s get bodies into every shop she went to regularly, the nearest supermarket, the gym she visited. Let’s take a good hard look at friends and workmmates. Interview all the neighbours again.’

‘Phil reckons he came prepared.’ Thorne picked up the post-mortem report that Hendricks had delivered the previous afternoon, flicked through it. ‘I’ve got a feeling he’d been “preparing” for a while.’

Brigstocke groaned. ‘How bloody long have I been doing this?’ he said. ‘And yet hearing stuff like that still depresses me.’ He eased himself up from Thorne’s desk and walked to the window. ‘I mean, I’m not saying it would be any better if her old man had caught her playing away from home and smacked her over the head with something. I know she wouldn’t be any less dead. But Jesus…’

‘It should depress you,’ Thorne said. ‘When it doesn’t-’

‘I know, time to retire.’

‘You turn into Trevor Jesmond.’

Brigstocke smiled. He picked up the piece of paper that had been spewing from the printer when he’d walked in. He looked down at the list of seven names. ‘This anything we should be looking at?’

‘Don’t see why,’ Thorne said. ‘Garvey died in prison three years ago.’

Brigstocke flapped the sheet of paper, as though he were fanning himself. ‘Just one of those freaky things.’

The DCI nodded his understanding. The pair of them had worked a case only a few months before in which a man had been beaten to death in front of his family after confronting a noisy neighbour. It transpired that twenty years earlier, and only two streets away, exactly the same thing had happened to that victim’s father.

‘One of many,’ Thorne said.

As it turned out, with a briefing that overran by twenty minutes and a Crown Prosecution Service lawyer who refused to get off the phone, lunchtime would have been a tricky time for Thorne to get away. But by then it did not matter: Louise had already called to say that she would be making her own way back to the flat. That she felt OK and needed to get out.

Driving back at the end of the day, Thorne felt nervous, as though he and Louise had had an argument. He ran through the conversations they might have when he got home, but they all went out of his head the moment he stepped into the silent flat. When he saw her lying on her side in the darkened bedroom.

‘It’s OK,’ she said. ‘I’m not asleep.’

It was only eight o’clock, but Thorne got undressed and climbed in beside her. They lay still for a while, listening to a motorbike revving up in the street outside, and a song Thorne couldn’t quite place drifting down from the flat upstairs.

‘Do you remember the Garvey killings?’ he asked.

She grunted and he wondered if he had woken her up, then she said, ‘I was at college, I think. Why?’

Thorne told her about Susan Sharpe. How a mother and daughter had been murdered, fifteen years apart. It was quiet now upstairs and Thorne still wasn’t sure what the song had been.

‘You’re doing it again,’ Louise said. ‘Trying to make me feel better.’

‘I wasn’t, I swear.’

‘And all you’ve succeeded in doing is making yourself feel old.’

Thorne laughed, for the first time in a few days. He pushed up close behind her and slid his arm across her stomach. After a few seconds he felt awkward and began to wonder if she would want it there, so he took it away again.

As per the standard system of rotas and rest days, Thorne spent seven Saturdays out of every eight at home. Normally, a Saturday morning would be taken up with sleeping far later than usual, nipping out for a newspaper then coming home for a gloriously unhealthy breakfast. Since Louise had come into his life, these were no longer always solitary activities, and thankfully the same was true of the sex, which could occasionally be squeezed in between the fry-up and Football Focus.

This Saturday, two days after Emily Walker’s murder, all rest days had been cancelled and overtime approved where necessary. Thorne sat in his office at Becke House, not looking through statements, ignoring the reports on the desk in front of him, wondering instead if the possibility of sex had now become remote.

When would it be all right to talk about it? Just how much of a self-centred bastard was he being, even thinking about it?

He looked over to the desk opposite, where Yvonne Kitson was working considerably harder than he was. She had been taken off a domestic murder that was all but done and dusted, and drafted in to bolster the top end of the team. Thorne was grateful to have her on board. Kitson was one of the best detectives they had, her achievements that much more impressive considering her circumstances, past and present. For several years she had been a single mum of two, her marriage having collapsed after a messy affair with a senior colleague that had also resulted in her formerly smooth progress through the ranks coming to a shuddering halt.

She glanced up from her desk, saw Thorne looking. She dropped her eyes again, turned a page. ‘What?’

Once, when neither had been laid for a while and it was debatable which was the more drunk, there had been the mildest of flirtations between the two of them, but they were long past that.

‘Saturday,’ Thorne said.

Kitson scoffed: ‘Never mind the bloody Tottenham game, or a morning under the duvet with Louise, or whatever you were thinking about missing. Some of us should be watching our sons playing rugby. I’ll have to be even more of a taxi service than I am already to make up for this.’

For a few moments, Thorne thought about telling her what had happened to Louise, getting a female perspective on it. But he just smiled and went back to the reports in front of him.

A minute later, a ball of paper bounced off his desktop and on to the floor. He bent to retrieve it and stared at Kitson. She shrugged, denying all knowledge.

Thorne unwrapped what turned out to be a transcript of that morning’s calls to the Incident Room. The published E-fit had generated a good deal of attention, and while the Press Office was handling the understandable media interest, the team itself had to deal with any information from the public. Thorne and Brigstocke had clearly underestimated the extent to which the picture would inspire some of the city’s more community-minded nutcases.

‘I wouldn’t mind coming in,’ Kitson said, pointing to the sheet of paper in Thorne’s hand, ‘if I didn’t have to spend all morning sorting through that shit.’

‘Got to be done, though,’ Thorne said.

They all knew it. Everyone on the team routinely joked about procedure and bitched about paper-pushing, and 99 per cent of the time, with a primary lead as shaky as their E-fit, nothing would come from this kind of work, but you had to double- and triple-check, just in case. Nobody wanted to be the one who missed the vital piece of information tucked away in a long list of crank calls. The clue hidden in the crap. In an age where the inquiry into the inquiry was commonplace, arse-covering had become second nature. It began before the victim was cold and would continue until the judge’s gavel came down.