‘How well did you really know her, Tyler?’ asks Lucas quietly.

I can hear something vaguely accusatory in his tone, and I don’t like it.

‘Well enough,’ I tell him. ‘She’s not involved in this, Lucas. She’s…’ I pause. ‘She was a really good person.’

‘I know, I know, but-‘

‘But what? Leah’s dead, for Christ’s sake. She was murdered. How can she have been involved?’

He sighs. ‘Listen, Tyler. How long have I known you?’

‘A long time,’ I admit reluctantly.

‘Exactly. You’re my friend. I respect your judgement. I know you really cared for Leah, but I’ve got to be honest with you…’ He stops and fixes me with an intense stare. ‘There’s something wrong.’

I open my mouth to argue, but something stops me. Instead, I sit back in the seat and listen. I think back to the night we met, how much I felt for her even then, and I feel a knot forming in my stomach.

‘You came to me yesterday asking for information about Leah,’ Lucas continues, ‘which means you weren’t entirely sure about her yourself. Also, the DVD shows quite categorically that she wasn’t murdered last night or early this morning. She was killed late on Wednesday.’

I wipe a hand across my brow, not sure what to think. The three weeks we spent together were some of the happiest of my life. For once, everything felt absolutely right. I can’t bring myself, even now, to believe that it was all an act on her part.

‘So, what is it you’re saying, Lucas?’

‘That it’s possible she was working for who-ever’s set you up. That maybe she was used to lure you in, but was more expendable than she thought. She was used, then killed, to seal your co-operation.’

‘But if I didn’t see her yesterday, who was it who lured me to that house? And who did I wake up next to this morning?’

‘I have no idea,’ he admits, with a weary shrug. ‘Neither, unfortunately, do you. Remember the anagram. Leah Torness. She’s not real.’

I still think there must be some mistake on the timing shown on the DVD, because I know who I saw this morning, and I’m absolutely positive it was Leah. The body shape, the jewellery, the tattoo. They were all hers. But I don’t press the argument. There’s no point.

Lucas sighs. ‘All right,’ he says, ‘we’re going to have to look at things from another angle.’

‘How about starting with Ferrie?’ I suggest, forcing myself to start thinking properly again.

‘Good idea.’

He pauses for a moment, mulling things over. I let him get on with it. He’s the detective, after all. As he thinks, he doodles on a giant desktop notepad. Finally, he lights a cigarette, blows a line of smoke towards the ceiling, and looks my way.

‘When you went to exchange the briefcases this morning, you told me that you were sent to one address but were immediately taken to another one?’

‘That’s right. A house just up the road.’

‘How far up the road?’

‘I don’t know. Fifty yards?’

‘Not far, then. Ferrie would probably have had to give them the location to send you to a while before you turned up.’

‘He did. The guy blackmailing me told me where to go an hour and a half before I got there.’

Lucas jots something down on the notepad, the cigarette dangling from his mouth.

‘Did the place where you finally met Ferrie look lived-in to you?’ he asks.

I shake my head, recalling its sparseness. ‘No, it didn’t.’

He nods slowly. ‘That’s what I thought. I can’t see a guy as nervous as you say Ferrie was swapping the cases at his home, or anywhere near it, so I reckon it’s safe to assume he lived somewhere else. We need to find out where.’

‘He was acting really paranoid this morning. It’s not going to be easy.’

‘Let me worry about that,’ he says sharply. ‘Now, when you were given the address, you weren’t told the name of the person you were going to see. Is that right?’

I nod. ‘They don’t know his real name. It’s one of the things they wanted me to tell them.’

‘Which gives us an advantage, because we do know it. Even if he’s covered his tracks, we’ll find him.’ He sends another pall of smoke skywards. ‘Trust me, I’m a detective.’

So I sit there drinking my coffee and trying without much success to relax for the first time today, while he continues to detect. And it soon becomes clear that Ferrie had indeed covered his tracks. When he hired Lucas, he paid for his services upfront and in cash, declining to give an address where he could be reached. But we’re lucky with the name. It’s comparatively unusual, and there are only four Iain Ferries on the electoral roll in Greater London. A combination of surfing the net and telephoning strategically placed contacts confirms to us that none of them has ever served in the military. A dead end? Lucas is undeterred.

‘You can find anybody if you want to,’ he tells me between phone calls. ‘As long as you know where to look. There’s all this shit with the Data Protection Act and how you’ve got to protect a person’s personal details, but the thing is, they’re held on so many different databases it’s impossible. And the security on those databases is worthless half the time. If you know a decent hacker, he’ll get inside and they’ll never have a clue he’s been there.’

And Lucas does know a decent hacker. He’s got the business card of someone with the bizarre name of Dorriel Graham who advertises himself as an IT security consultant. ‘This guy’s the best,’ he tells me, calling the number on the card.

While he’s not looking, I write down the number myself. You never know when skills like that may come in useful.

And come in useful they quickly do. Lucas gets him to hack into the Ministry of Defence computer systems. Now, given that the MOD are supposedly in charge of defending the realm, I would have thought this would be near enough impossible, but it seems some of their systems are more secure than others, and the database that contains the details of serving and recently demobbed soldiers is eminently hackable. Ferrie may have left the army some time ago, but the MOD still have a record of him, and within fifteen minutes of Lucas’s call a two-page document with photograph is coming through on his printer.

‘This’ll help us,’ he says, reading through it. ‘Ferrie might not be on the electoral roll or the Land Registry, but people close to him will be. See, it says here he was married in 1999 and that his spouse is a Charlotte Melanie Priem. There’ll be a record of her somewhere.’

His next port of call is the Register of Births, Marriages and Deaths, a database that any member of the public is allowed to access. Armed with the date of the marriage and the names of the couple, he quickly finds that it ended in divorce in December 2003, on the grounds of Mr Ferrie’s unreasonable behaviour. No further details of what he did are given, but we don’t care about that. What we care about is the fact that the petitioner, Miss Priem, gives a flat in Enfield as her permanent residence. A check on the Land Registry shows that she still owns the flat, and a quick call to his contact at BT gets Lucas its landline number. It’s all very easy if, as he says, you know where you’re looking.

‘Let’s hope she’s in,’ I say.

He shrugs. ‘It doesn’t matter if she’s not. Chances are she’ll have a mobile registered to that address – I’ll just get hold of that. More importantly, does she know where he is?’

He lights another cigarette and calls the number.

Ten seconds later, Lucas embarks on some time-honoured patter. ‘Hello, Mrs Ferrie? Oh, I’m sorry, Miss Priem. I apologize for bothering you but I’ve been trying to locate your ex-husband.’ He tells her he’s a former soldier who served with Iain, and wants to invite him to a regiment reunion. Something about his manner – all chirpy, cheeky charm – clearly works for the ladies because within seconds they’re chatting like old friends. From the way he’s speaking, it sounds like she’s firing off a lot of not very flattering comments about her husband, which comes as no surprise. ‘Oh, that’d be great if you could do that, Charlotte. You’re very kind.’ He winks at me as he speaks and gives the thumbs up. ‘Thanks, that’s really helpful… No, to be honest, I didn’t get on with him that well, but I’d feel bad if he didn’t get an invite, and there are a couple of people who do really want to see him.’

There’s a short pause, then Lucas scribbles something down on the notepad. It looks promising.

‘I agree,’ he says into the phone, ‘if he was like that, then it’s inexcusable… No, you should never do that… That’s right, you couldn’t have known…’ He rolls his eyes at me. ‘Don’t I know it? We always find these things out too late… What do I do now? I’m a forest ranger… Yes, I’ve always loved the outdoor life. Now, if you’ll excuse me, I really have to go… Yes, thanks… thanks… Definitely, if I get the time… OK… Bye.’ He slams the phone into the cradle. ‘Jesus, I’d be behaving pretty fucking unreasonably if I had to live with her. She wouldn’t shut up.’

‘But we’ve got what we wanted?’

He nods, ripping the paper containing the address from the notepad and stubbing his cigarette out in the ashtray. ‘Yeah, she saw him three months ago. He was living in a flat in Southgate. She thinks he’s still there. A place called Frobisher House.’

I could do with a longer sit down, but it’s already gone five, about two hours since Snowy was murdered in broad daylight, and pretty soon the police are going to be phoning Lucas about it.

I think he’s thinking the same thing because we get to our feet simultaneously and three minutes later we’re in his BMW and heading north on the Holloway Road.