I decided to start on the ‘unsolved stranger crimes’ already in the computer system: it looked less daunting than the scatter of dusty binders.

I scrolled down through countless unsolved attacks on women, mostly sexual.

London’s rape statistics were one of the few things that stayed with me from my training. Every day, the Metropolitan police force received six reports of rape and twenty-one reports of sexual assault. About ten per cent of these attacks were carried out by someone unknown to the victim: that’s about four stranger rapes per week. Judging by these cases, most ‘stranger’ rapists tried to force anal intercourse on their victim too.

The sheer scale of random, seemingly arbitrary violence scared me: the city is full of men who despise women. Every time I concluded that an unsolved rape/attack on a woman couldn’t be linked to Marion’s murder, I wrestled with more guilt. It felt like I was walking away from their cries for help, leaving them lying in the dirt where they’d been violated.

Yet I knew I had to be brutally selective, or we’d wind up with more suspects than police officers.

Of course, detectives all over London had already been busy making connections. The first serial attacker to catch my eye was the so-called Night Stalker, who operated in South East London. Described as black and in his thirties, this man expertly broke into the homes of old women living alone in the middle of the night. He raped them at knifepoint and stole anything valuable to hand.

Marion was twenty-three. She hadn’t been sexually assaulted. As far as we could tell, her killer hadn’t broken in. Nothing had been stolen from the scene. I ruled him out.

Links had been made between four attacks near train stations in West London. The attacker threatened his victims with a knife, led them to covered wasteland then raped them. Marion’s flat was five minutes’ walk from Clapham Junction train station. Had he been casing out that station, only to discover a complete lack of deserted areas to strike? He noticed Marion. He came back to wait for her again, and again. One day he followed her, found out where she lived, did nothing. He bided his time, confirmed her daily routine over days and weeks, like an assassin. Then one day he waited near her home. Somehow he distracted her as she opened the front door, forcing her at knifepoint to unlock the flat and walk up the stairs. She fought back. He lost it and went berserk, then fled the scene.

I realised that the same case could be made against every stranger rapist in any file. I had to find something more tangible, a connection more damning and evidential. I trawled and trawled that eye-bleeding green text until the end, failing to settle on a single connection.

I printed out the list of ‘Currently Misplaced’ paper case files and cross-referenced it with the folders sprawled across the empty office floor. I realised that one new file had turned up since this stock take. I spotted it immediately – the one not caked in dust. The courier’s packaging confirmed it had been delivered from Plumstead police station in East London that weekend.

As I opened the box file, I felt an icy frisson of fearful anticipation: no one else had seen this ‘unsolved stranger attack’ paperwork. What if he was in here? What if I found him? I told myself to be extra-vigilant. I reminded myself that he liked to use a knife and attack women in their homes. I thought of the messages Marion’s spirit had communicated to me: banging the door, appearing to me on two streets near the crime scene. I pushed it all to the front of my mind, ready to snag on any link or connection in these files.

The first case to catch my eye, because of its name, was the Green Chain Attacks. To my relief, the Green Chain turned out not to be some soothingly-coloured yet gruesome martial arts weapon but a series of connected parkland walks near Plumstead. Over the past four years, a lone male had attacked over seventy women on the Green Chain. His crimes had escalated from beatings, to sexual assaults to rape. He sometimes used a ligature, more often a knife.

Descriptions of the suspect were vague because he always wore a balaclava or a mask. As I’d discovered in most other linked sex attacks, witness estimations of weight and height varied wildly. He’d been described as five foot six and broad, six foot two and slim, and just about everything in between. But DNA evidence proved it was the same man.

I looked at the other odds and ends in the box file. An intelligence report from two months ago caught my attention. A couple with an address at Winn Common – part of the Green Chain expanse – had phoned police describing a man ‘hanging about’ in their neighbour’s back garden, apparently spying on a young blonde woman ‘who often walks around her flat semi-naked’. The husband went out and kept an eye on the suspect until a police patrol arrived.

‘I bet he did,’ I thought.

When police quizzed the loiterer, he insisted he was merely taking a walk and had stopped for a piss. He gave his name and address: Robert Napper, 126 Plumstead High Street. The officer wrote: ‘Subject strange, abnormal. Should be considered a possible rapist, indecency-type suspect.’

I took a deep breath and stopped my mind skidding off in too many directions. Had this man progressed from attacking women on the Common to breaking into their homes? Surely we should at least cross-reference his DNA with the Green Chain victims?

I rifled through the file’s remaining contents. Intelligence reports identified other local weirdos skulking around parks and commons, spying on sunbathers, flashing and wanking behind bushes. Other notes identified couples and gay men and their repeated attempts to initiate ‘stranger sex’ in public. I was beginning to realise that London’s green spaces gave a whole new meaning to the term, ‘Parks and Recreation’.

Then I found a handwritten note. It was dated almost two years ago – October 1989 – but someone clearly believed it deserved attention. ‘At 9.10 a.m., Mrs Maureen Napper of 19 Raglan Road SE18 rang to say that her son, Robert, 26, of basement flat, 126 Plumstead High Street, told her he’d raped a woman “on the Common” two months earlier, in August. The police sergeant on duty checked the records and found no report of an attack or rape on Plumstead Common, or any connected green spaces, during August or the previous two months.’

My hands trembled. It was that man Robert Napper again, who they’d recently found in the garden of a woman’s flat. Allied to this rape confession, his MO surely made him a suspect for the Green Chain attacks, if not Marion’s. I asked Mick what the procedure was for pursuing leads like this: ‘Just ring ’em up,’ he said. Soon I was on the phone to a duty officer at Plumstead. He agreed to check the log book of all recorded crimes for August 1989. He confirmed there had been no reported rapes or sexual assaults on any Commons on their patch that month. He checked July and September: same result.

… on the Common. Did Napper even mean Plumstead Common? He could have struck at another Common: Clapham Common? I checked the A to Z: there are Commons all over London, dozens of them.

It was already after six. As Mick got up to leave, I asked him to run Napper through the police computer. ‘Robert Clive Napper, born February 25, 1966, one criminal conviction: possession of an airgun in a public place, 1986.’

‘Sounds like a menace, at least to the local rabbit population,’ he laughed.

I had a feeling about Napper. But I didn’t want to present it half-cocked at Monday morning’s briefing. I needed to connect Napper to the rape he’d boasted about. If I pinpointed that rape – the rape he’d confessed to his own mum – we could cross-reference it forensically with any DNA and prints found at 21 Sangora Road. This new twist made me realise I had to go through all of the remaining paperwork – and back again through the cases on the computer – to cross-reference all rapes on London Commons between June and September ’89.