The air in the incident room fizzed, charged, electric.

‘Last November, she saw Karen and Peter enter a shed in the grounds of the home where Peter has a desk. Fifteen minutes later, Karen emerged, as Bethan put it, “dishevelled”. She’s convinced that they’d had sex in the shed.

‘A few days later, she confronted Karen, who denied having any sort of sexual relations with Peter Ryan, insisting that they were just good friends. Bethan claims she believed her, which is why she hasn’t mentioned any of this before.’

Shep swelled with almost unbearable self-satisfaction, a raging blush igniting both cheeks.

‘So, Peter Ryan and Karen Foster were having sex late last year, just months after Peter had married Marion. The challenge now,’ he declared, all-conquering, Churchillian, ‘is to prove that Karen Foster acted upon her obsession with Peter Ryan, by killing Marion.’

‘Obsession?’ scoffed McStay, holding up a VHS tape. ‘If Karen had some sort of homicidal hatred for Marion, then why did she attend their wedding in Ireland last summer? Karen is on this video, having a great time. Certainly looking nothing like the woman scorned.’

‘I’m sure she presented well to camera,’ sniffed Shep.

McStay wasn’t done: ‘It’s worth noting that Peter Ryan is still staying with Marion’s family up in Enfield. They don’t suspect him of any wrongdoing whatsoever. They’ll be shattered by this accusation.’

That punctured the euphoria. The O’Learys had been almost saintly in their dignity and patience. They really didn’t deserve a sordid sub-plot.

Shep set about resurrecting the mood: ‘DS Barratt. Tell us please what you discovered from Peter’s best friend?’

‘Not his best friend, Guv, his best man, who comes from his home town in Ireland but lives in North London.’

Shep blinked impatiently.

‘He told us that Peter and Marion were planning to move to Ireland in the autumn to start a family. They hadn’t told their families yet: they were waiting until Marion was pregnant.’

Shep decided to editorialise for anyone failing to keep up: ‘Karen got wind of these plans. Time was running out for her and Peter, the man she loved.’

I wondered if, on the sly, he read his wife’s Mills and Boon books.

Barratt remained deadpan: ‘Well we need to find out if Karen actually knew about these plans first, Guv.’

Shep ignored him and turned to the last of the four teams. Maurice, the younger of the pair, spoke up: ‘We found out something from Pam Foster, Karen’s mother. She attended Clapham police station on the night of the murder while Karen and Peter were making their initial statements. When they were finished, she offered to drive Peter to Marion’s parents’ home. He told her that he couldn’t face them, and stayed the night at the Foster family home in South London, on the couch apparently.’

Shep raged: ‘So we let the two prime suspects spend a night together, to get their stories straight. And we didn’t even know about it? Christ almighty.’

Shep wasn’t as rattled as me. I realised I’d forgotten to read those initial statements made by Peter and Karen on the night of the murder. They’d given these statements raw – before their all-night, post-murder conference. If they were going to slip up, it would be in there.

I scolded myself for making such a basic error. As soon as this briefing ended, I’d gut those initial statements. But at that moment, I faced a more immediate dilemma. I realised that if I flagged up my stranger suspect, Robert Napper, Shep would probably destroy me.

He was coming to me next, soft soaping the old guard in the meantime: ‘Of course, we still have to keep an open mind about other potential suspects. Peter and/or Karen could have roped in an accomplice. And, of course, the Lone Wolf Killer line of enquiry remains open. Lynch, did you come across anything we need to check out?’

My face burned. I had to flag up Napper. I had to. If I chickened out and Napper struck again, I’d never forgive myself.

‘Yes, actually I think there is someone we need to check out, Guv,’ I said, my voice shaking slightly. I felt like a noise pollution officer shutting down a banging party.

‘Did a Lone Wolf jump out of the paperwork and bite you?’ he laughed.

The sniggers sucked the last drop of moisture from my throat.

I told them about the Green Chain Attacks. I pointed out the obvious connections to Marion’s murder: the suspect had escalated, he’d used a knife, he’d targeted women both in public spaces and in their homes.

So far, so plausible. I then hit them with the notes that put Napper in the frame: the first written by the patrol officer who caught him hanging around a woman’s back garden (‘abnormal, rapist, indecency type’), then the note which described the rape claims he’d made to his mother.

That last revelation shocked even these gore-hardened hounds.

‘Christ, what sort of pervert would boast about a rape to his own mum?’ said Shep to murmured agreement.

‘So what sex attacks on local Commons did you find to match his confession?’ asked Shep.

‘Well, none, Guv, but I think it’s worth checking him out.’

‘What’s his form?’ asked Shep, irritably.

‘Er, not much,’ I mumbled, my cheeks sizzling like fried rashers, ‘possession of an air rifle in ’86.’

I sounded half-hearted, it sounded half-arsed.

‘And?’ said Shep, chewing his lip.

I shrugged: ‘That’s it,’ I said, trying not to swallow my own voice box. Nobody moved a muscle.

Shep spoke patiently, as if to a child: ‘So he’s got no form for a crime like this, and the note from his mother is from nearly two years ago and doesn’t match any reported crime?’

‘That’s correct, Guv, but it’s not every day a mother shops her son for rape. She must know more. He must be some sort of deviant.’

‘I think you’re clutching at straws, Lynch. I mean none of the Green Chain Attacker’s victims were stabbed, correct? As far as I know, the attacks all took place in public places. I just don’t see the connection.’

‘He may have struck in someone’s home. We just don’t know yet. But in conscience, Guv, I had to bring this guy to your attention. This nutter is the only person I could find who’s on the loose and capable of this level of violence. He’s escalated to this. He’s worth checking out. That’s all I’m saying.’

Shep shook his head: ‘This is a weirdo who lurks in the bushes at night and jumps on random women. You think he suddenly took a bus to Clapham, saw Marion and decided to stab her up?’

Childhood humiliations flooded through me. Can someone else go in goal now?

‘He might have been stalking her?’ I heard my feeble voice plead.

Shep smiled: ‘Do you think he brought his airgun with him, on the bus?’

That got a laugh.

‘Lynch, get the officers at Plumstead to check him out. Can you stick to solving this case for now?’ he said with a mixture of disgust and pity. I hoped to God he’d let it go.

Shep surveyed his flock slowly, dramatically. He leaned forward and spoke quietly, conspiratorially: ‘Let’s be bold and run with the idea that Karen Foster murdered Marion.’

Hang on, my mind protested, the pathologist said it had to be a man. My memory flashed back to Karen that night, convulsing as I questioned her. That can’t have been an act.

But Shep had the blinkers back on: ‘Okay, so based on what we’ve got, Karen takes the afternoon off work. She meets her accomplice. She either brought the weapon with her, the accomplice brought it or they went somewhere to pick it up. They drive to Marion’s home at Sangora Road. She and the accomplice park where they can see the far end of the street and the front door. Marion turns into Sangora on foot just after five forty p.m. Karen and the accomplice meet her at the bottom of the steps to 21. One of them is carrying a bag containing the weapon and a change of clothes.