Later today, I was due to make a statement to the investigating team. By re-examining the murder scene, perhaps I could offer up a few fresh insights or theories; make a good impression. I needed a senior officer to spot me and think that I was worthy of championing; to take me under his or her wing.

I took a short cut across Wandsworth Common, ignoring the tarmac pathways. London’s green spaces seemed so orderly and controlled to me: the opposite of nature. It’s a wonder there weren’t signs saying, ‘Keep Off The Grass’. As I trod the dewy sward, I let my mind drift off-road too. After this morning’s chilling encounter, I needed to open myself up to all possibilities. It was time for a logic amnesty.

One fact felt indisputable: Marion’s attack on me hadn’t been a dream. When she came to me in the flat, I’d been wide awake, albeit a bit pissed. I could see and hear everything around her in the streetlight orange tinge – the furniture, the traffic outside, the slamming door. I’d felt her breath on my face.

And, three years earlier, I’d felt Meehan’s cold, gloved hands strangling my throat.

I didn’t believe in ghosts, spirits, religion, the supernatural or any of that stuff. But the most obvious explanation for what happened last night – however crazy – was that the spirit or ghost of Marion Ryan had come to me. A few hours earlier, I’d been physically close to her recently murdered body. Three years ago, Meehan came to me at Tullamore General Hospital as I slept upstairs from the basement morgue hosting his fresh corpse. I had to ask the question: did my proximity to a body that had just met a violent death somehow open up a telepathic pathway between us? And if so, what were their spirits trying to tell me? And how could they get inside me?

The naked malevolence of Meehan’s assault didn’t seem to say much, apart from he wished me harm. But while Marion’s attack felt every bit as threatening, there was something about the encounter that made me think she had been trying to tell me something. That slamming door. What did it mean? I had to get to Sangora Road and see if something snagged on my mind.

As the grass of the Common gave way to concrete, a rancid stench invaded my senses. I checked both soles, located the soft wet dog shit wedged between the grips of my left shoe and declared the logic amnesty over.

As I rubbed my shoe against a grass verge, I tried to come up with a more believable solution. Meehan throttling me had been a graphic hallucination. After all, I’d just ingested enough tranquilliser to poleaxe a sadhu. Marion’s apparition was a result of post-traumatic stress – or post-traumatic Shiraz, as Aidan put it. Seeing her wound-covered body last night had obviously affected me more than I’d realised.

Sangora Road had already recovered its leafy, anonymous poise. On one side, a road sweeper clanked along grudgingly. On the other, a couple of suits made breakneck progress towards Clapham Junction train station. Ahead of them, a racket of rotund school kids swore loudly, smoked and spat. I wondered what Tullamore’s own Jesuit terrorist Father Devlin would give for ten minutes in a locked room with that lot, and how much I’d pay to watch.

Across the road from number 21, the press pack swarmed, keen as hyenas. I counted nine still camera lenses, presumably all jostling for the same shot. I couldn’t help thinking: what a waste.

As I cast them my most contemptuous glare, a morose Northern voice stopped me in my tracks.

‘Where do you think you’re going?’

Clive ‘Overtime’ Hunt was one of the less offensive nicknames earned by my beat partner over the years. Colleagues would plead with me: ‘Find out what he does with all the money?’ He simply couldn’t say no to overtime and must have worked at least seventy hours a week, every week.

‘You did go home at some stage, Clive?’

‘Oh yeah. I got back here at eight this morning. Easiest gig going this, standing at a door.’

‘Exciting too,’ I said, ‘so what’s been happening?’

‘They removed the body about two hours ago. Forensics are still working, so don’t cross the tape, obviously,’ he drawled, the irritating tit.

He unlocked the internal door to the flat, then almost ceremoniously pulled it open.

‘Thanks, Clive,’ I said, ‘but that’s really not necessary.’

‘Oh but it is,’ he said, ‘it’s one of those fire doors, spring-loaded to close itself, except it’s been sprung too tight.’

‘Oh,’ I said, putting my palm against the half-open door. Just like last night, it pressed back hard.

PC Know-It-All’s furniture-and-fittings briefing wasn’t over yet: ‘Clever things, these fire doors; at a certain temperature they expand and seal the gaps, blocking out flames and, even more important, smoke. Smoke inhalation is actually the biggest killer, you know.’

‘Fascinating,’ I said, turning on the first step, then letting the door go so it slammed behind me.

I imagined Marion coming up these stairs. She had her post, keys, handbag and jacket over her arm. Whoever killed her was either in the flat already, or had met her at the front door as she returned from work. The idea that someone was already inside felt less plausible – there were no signs of a break-in. Only she and Peter had keys. If she let her killer in, then she must have known him.

Ninety-eight per cent of murder victims know their killer.

He followed her up the stairs, launching his attack from behind. But if it was Peter, why would he stab her to death on the landing? He could have killed her in any of the rooms, in a variety of ways, at any time of the day or night, silently and without leaving evidence or causing a commotion. It just didn’t stack up. Unless it had been a crime of passion: one of them had been having an affair, confronted the other. Peter had lashed out in a blind rage. It’s always the man, isn’t it?

My mind turned to Peter and Karen finding the body. I imagined myself as Peter coming up the stairs. I was about the same height – five ten – so I stopped at the spot where he would have seen Marion’s body on the landing.

Karen wouldn’t have seen the body yet. She was five foot four, tops, and must have been at least a step below Peter, if not two. He called Marion’s name and went to her body.

Karen told me she’d been the one to check for signs of life. That’s how she got Marion’s blood on her hands. Had Peter made any sort of check first? If not, then why not? Did he already know she was dead? I couldn’t be sure if this meant anything, but made a mental note just in case. I’d mention it later to the investigating officers, show them I had solid detective potential.

I walked up to the police tape and exchanged a nod with the forensics who were tweezing every inch of the landing.

There seemed to be very little blood on the carpet and walls, considering all the wounds Marion had suffered. Either she had died quickly – blood stops flowing when you expire – or most of her wounds were superficial. I wondered if I’d get a look at the pathology report.

The window on the landing overlooked a flat roof below. Someone could have climbed onto that roof and scrabbled up the rooftiles to this window. I unfastened the latch and opened it as far as it could go: about four inches. It looked new and hadn’t been forced. The killer didn’t get in through here.

Cool air rushed my face, voodoo lurking in its slipstream.

After three hours, they close the window, to ensure that the spirit doesn’t return.

She usually gets home before six … we got back just after nine …

Had I been standing here as her spirit returned, hungry for vengeance?

My eyes followed the blood streaks along the wall. I wondered what would happen to the flat now. Surely Peter could never come back to live here, guilty or not. Who would paint over the blood? Would future prospective tenants be told of the horror that had taken place, here on these stairs?