They had not spoken about the incident since.

Much had changed for both of them since a case that had taken its toll on each in different ways and had ultimately cost Thorne’s father his life. They did not speak about that, either. Though it was always there, a shadow between them, they just got on with taking the piss, same as any other two coppers, despite the differences in age and experience.

Negotiating the crowds, Thorne talked her through the inquiry; the link between two series of murders fifteen years apart. She remembered the Garvey case very well, she told him, having worked for a number of years with the SIO. She had been close enough to have watched a few of the early interviews.

‘He never said why he did it, did he?’ Chamberlain said. ‘Like Shipman. Never gave any reason for it. They’re always the worst.’

‘Maybe there wasn’t a reason. Maybe he just liked it.’

‘There’s usually something, though, isn’t there? With most of them. The voice of God telling them to do it. A message from the devil in a Britney Spears song. Something.’

‘Well, this one’s certainly got a motive,’ Thorne said. ‘Or thinks he has. He wants us to know exactly why he’s doing it.’

‘OK, forget what I said. They’re the worst.’

They carried on towards Tottenham Court Road, crossing Oxford Street at Chamberlain’s insistence so they could walk in the sunshine. He told her about the search for the three missing sons of Raymond Garvey’s original victims, and about the phone call from Pavesh Kambar.

Thorne had done some checking and discovered that Nicholas Maier had written a book about the Garvey case that was published a year before Garvey’s death. He had picked up a copy of Battered – The Raymond Garvey Killings from his local Waterstone’s in Camden before catching the Tube. On first glance, it looked much the same as the ones he had bought online. The same pictures, the same semi-salacious blurb on the back of the jacket. He fished the book from his bag and showed it to Chamberlain.

‘When are you seeing him?’ she asked.

‘Monday,’ Thorne said. ‘He emailed me back from his “lecture tour” in America. He gets back tomorrow.’

Chamberlain pulled a face.

‘I know. They teach this stuff in universities over there. Serial Killers 101, whatever. Said something in his email about it paying for his next holiday. Also said he’d be happy to meet me.’

‘I don’t like the sound of that.’

Thorne laughed, knowing very well what she meant. He was always suspicious of anyone who seemed overly pleased to see a police officer. It wasn’t his job to be popular.

‘I mean, I know you,’ Chamberlain said, ‘and even I’m not happy to see you.’

They crossed back over the road and cut down into Soho Square. Though it wasn’t exactly warm, there were plenty of people gathered on benches or sprawled on the grass with books. They squeezed on to a bench next to a cycle courier who was finishing a sandwich. He got up and left before he’d swallowed the last mouthful.

‘So, what do you need?’ Chamberlain asked.

‘We need to know where this bloke comes from. It’s looking very much like he’s Garvey’s son, so let’s start with trying to find out who his mother is. It doesn’t sound like she was in contact with Garvey.’

Chamberlain was still holding the book. She lifted it up. ‘Why don’t you ask your new best friend?’

Thorne took it back. ‘I’ve skimmed through and there’s nothing about any son in there. I think Anthony Garvey made contact with him after his father had died.’

‘You think he wants Maier to write another book? Go into all this brain tumour stuff?’

‘I’ll find out on Monday,’ Thorne said. ‘Meanwhile, you can start digging around, see what you can come up with. All the descriptions put him at thirty-ish, so he was born fifteen years or so before Garvey started killing. You up for it?’

Chamberlain nodded. ‘Well, this or the gardening? It’s a tough choice.’

‘AMRU not keeping you busy, then?’

‘They couldn’t afford me and the hypnotherapist.’

‘Sorry?’

‘The brass thought it would be a nice idea to try some regression therapy on a few witnesses, see what they could remember.’

‘Right. “I think I used to be Marie Antoinette”, all that.’

‘They reckon this bloke got some witness to come up with a number plate she’d forgotten. I don’t know…’

‘Jesus.’ Thorne never ceased to be amazed at what people could waste resources on in an effort to make a splash. ‘Things are rough when you get bumped off a case for Paul McKenna.’

Chamberlain smiled. For a while they sat in silence, watched the comings and goings. A skinny, rat-faced teenager was moving among the groups on the grass, asking for money, meeting each refusal with a glare. A chancer. He looked at Thorne, but showed no inclination to try his luck.

‘Someone who’s definitely not happy to see you,’ Chamberlain said.

She asked about the hunt for Andrew Dowd, Simon Walsh and Graham Fowler, and why they had not turned to the media for help. Thorne told her what Brigstocke had said about the emphasis being on catching their man, and Kitson’s theory that they were using Debbie Mitchell as bait.

‘Nothing surprises me,’ Chamberlain said. ‘It’s all about the result, right?’

‘They’ll get one they really don’t like if they’re not careful,’ Thorne said. He explained that they were doing their best to trace the missing men through conventional channels – credit cards, mobile-phone records, good old-fashioned donkey-work – and getting nowhere. ‘Dowd’s away trying to find himself, if his wife is to be believed. The other two are off the radar altogether. Homeless, maybe; drifting for sure. All of them have got… problems.’

‘Sounds like they’ve all got one very big problem.’

Thorne nodded, watched the rat-faced kid arguing with a community support officer who was trying to move him on. ‘It’s not really a shock, though, is it? That they’re all screwed up in one way or another.’

‘We all carry our pasts with us,’ Chamberlain said.

‘Yeah, well, maybe that hypnotherapist’s on to something.’

‘Carry them around like bits of crap in our pockets.’ She sat very still, patted the handbag on her lap. ‘We know that better than most, don’t we?’

Thorne didn’t look at her. The skinny teenager was wandering away now, shouting and waving his arms. The CPSO laughed, said something to one of the people lying on the grass. ‘How’s Jack?’ Thorne asked.

‘We had a cancer scare,’ Chamberlain said. ‘Looks like we’re OK, though.’ She glanced across at Thorne and spoke again, seeing that he was struggling for the right thing to say. ‘What about Louise? You know, I’m not convinced you haven’t been making her up.’

Chamberlain and Louise had never met. Thorne himself had not seen Chamberlain in over a year, although he made a point of trying to call her as often as he could. He felt oddly guilty.

‘She’s busy,’ Thorne said. ‘You know how it is.’

‘Two coppers together. Always a big mistake.’

It suddenly struck Thorne that he had no idea what Chamberlain’s husband did; or used to do, before he retired. There was no way to ask without making it obvious. ‘You’re probably right,’ he said.

They sat for another minute or two and then, with a nod to each other, got up and moved through the square, walking out towards Greek Street and into the heart of Soho.

‘I’ll send over all the stuff later,’ Thorne said. ‘And a copy of the original Garvey file.’

‘A nice bit of bedtime reading.’

‘What is it normally, then, Catherine Cookson?’

She flashed Thorne a sarcastic grin, then slowed to stare through the window of an arty-looking jeweller’s. She leaned in close, trying to make out the prices on the labels, then turned to Thorne and said, ‘Thanks for this, by the way.’

‘It’s not a problem.’

‘I know you could have found someone closer to home.’