He disappeared round the corner. I assumed he was on his way to the murder scene. I couldn’t for the life of me imagine why. I peeked around the corner and saw him stride into the Roundhouse pub.

I didn’t know what to do next. If I walked past the pub, he might spot me through the window. Whoever he was meeting could pass this way. How suspicious would I look hanging about here? I had a sudden brainwave and speed-walked, Shep-style, back to Clapham Junction. I had remembered the short taxi rank at the traffic lights, opposite the entrance. Luckily, a single black cab sat there, light on. I jumped in, flashed my badge and told him to drive towards the Roundhouse pub. I explained that I needed him to park further up Sangora Road, facing the pub so that I could see who was coming in and out. And that I needed him to be quick.

He didn’t move.

‘What are you waiting for?’ I muttered.

‘I usually set the meter to night-time rates for surveillance operations,’ he announced cheerfully. I was scarcely in a position to negotiate.

‘Fine,’ I said, and he hauled his wheezing black beast into thick traffic.

The nagging voice in my head wondered why Shep was so confident that the CPS lawyer would back his decision to charge Karen. We didn’t have a firm sighting of her at the scene. The steel ruler’s modified blade may or may not have been the murder weapon: it was impossible to prove. There was no forensic evidence at the scene. Her affair with Peter was winding down. She hadn’t expressed a desire to kill Marion to anyone. And what about the male accomplice? No one had any idea who that could be.

Yes, Karen and Laura had lied about their whereabouts on the afternoon of the murder. Yes, Karen had lied about ending her affair with Peter in November last year. But none of this proved they killed Marion Ryan. Since day one, Shep’s obsession with Karen Foster had been all-consuming. What if McStay and Barratt and the Big Dogs were right all along? What if she was innocent?

As the black cab pulled up on Sangora, I ducked down. I didn’t have to wait long. Fintan emerged from the pub, blinking like Barabbas against the light. He raised his collar and set off up Strathblaine, battered leather satchel tucked firmly under his arm. Ten minutes later, Shep stepped out. He looked right, then left, directly at the taxi. He raised his arm to hail it, then registered that the light was off and started walking our way. I got down on all fours and told the driver to step on it to Clapham police station.

As I stared at the stippled grey plastic flooring, Fintan’s words outside Buckingham Palace that day pealed through my mind.

People in power want more power. They don’t serve the public, they serve their own agendas.

‘I’m off to win hearts and minds,’ Shep had said. What had he told Fintan?

The smarter ones recognise the power of the press, and use it to put pressure on their own organisation.

I jumped out at the police station, overpaid my overweight getaway driver and took a look at my beeping pager.

‘Lawyer says we haven’t got enough,’ read the group message from Shep.

As I’d suspected, the CPS brief felt we still lacked that single piece of irrefutable evidence – that elusive smoking gun – to charge Karen.

I marched back into the incident room, my mind made up. I was going to find out who killed Marion myself, once and for all. I had just scooped up the keys to 21 Sangora Road, when the receptionist walked in.

‘Donal Lynch?’

I nodded.

‘I’ve got a message for you to call Fintan.’

As I dialled his direct office number, I thought about mentioning that I’d just seen him in Clapham. That’d rattle the smug fucker.

‘Yeah?’

‘Fintan?’

‘Donal, I’m being arse-fucked by a deadline. Can it wait?’

‘I just got a message to call you.’

‘News to me.’

‘Oh. Okay. What time do you knock off?’

‘I’ve got, let’s see, twenty-six minutes to write the splash.’

‘Meet me after,’ I said, ‘I’ll be at the Roundhouse pub, near Sangora Road in Clapham. You know it?’

‘I don’t think so,’ he lied, ‘but I’m sure I’ll find it.’

I took the overground train to Brixton, bought what I needed on Electric Avenue and headed back to Clapham Junction. By the time I got to the Roundhouse pub, Fintan was waiting.

‘Didn’t even know this place was here,’ he smiled, confirming that you shouldn’t even believe his ‘hello’.

As the barman approached, I remembered one of my dad Martin’s favourite expressions – ‘Beer isn’t drinking’ – and ordered two large scotches. Lame liquor had no place in this enterprise.

‘As delightful as it is to see you, Donal, what are you after?’

I opened the palm of my hand, revealing a set of house keys.

‘Let me guess,’ said Fintan, ‘we’ve been invited to a swingers’ party?’

‘Try again,’ I said, ‘and think about where we are.’

‘They’re not … Holy shit. Isn’t it still a crime scene?’

‘No, they’re all done. And I’ve brought along a little something to help me prolong the experience,’ I said, opening my other hand to reveal an eighth of cannabis.

It must have been about nine when we walked out of the pub into the snug, muffled dusk. At the doorway, Fintan furtively removed the batteries from his pager.

‘What are you doing?’

‘Making sure no one can place me at the scene.’

‘What?’

‘These things send and receive messages via the nearest transmitter, which means they can work out your location. But not if you take the batteries out.’

I dreaded how paranoid he’d get after a joint.

We popped into an off-licence for a bottle of scotch, tobacco, a lighter and extra-long papers.

As we walked up the steps to 21, I felt giddy, high, fairground-scared. I suddenly understood the buzz that breeds serial burglars. The key clicked sweetly in the lock. An icy chill wafted my neck. I thought of Marion that evening, turning this key, Karen Foster fingering her blade in the gym bag, poised to strike.

I opened the door and stood aside for Fintan. He looked shaken, lost.

‘This is too weird,’ he said, his eyes restless, unsure.

‘Get in for fuck’s sake, before someone sees us.’

As we stood facing the flat door, I felt his helpless stare.

‘Just pretend it’s my place,’ I urged, stepping forward and sliding the key into the lock.

I pulled the heavy door open, leaned against its weight and nodded at Fintan to enter.

‘Why do I have to go first?’

‘Jesus just get in the fucking door!’

He faltered, then crept inside. I took the key out of the lock and followed him.

Something cracked me in the back.

‘Jesus,’ I cried out.

‘What?’ squealed Fintan.

‘That fucking door just tried to knock me out.’

I followed him up the stairs and turned on the landing light. The zap of familiar yellow instantly banished my nerves. Sure enough, Marion’s bloodstains had been painted over. I talked him through the crime scene, hoping my breezy tone would normalise this morbid tour.

‘So what now?’ he said when I’d finished.

I led him into the sitting room. He sat on the couch, bolt upright, tense. I plucked two glasses from the kitchen and poured large ones.

As he glugged greedily, I hoped one bottle of Glenmorangie would be enough. As I knew only too well, numb don’t come cheap.

We sat there in tortuously stilted silence. But that’s murder scene parties for you. Then I remembered the pot and set to work on a six-sheet Clapham Courgette, as they call them in that neck of the woods. Fintan took an almighty toke: we were both craving levity of any kind. Soon I was entertaining him with imitations of Peter Ryan, Karen Foster and Shep, and had him slapping his knees in helpless hilarity.

I got back from the loo to find Fintan comatose. I dipped the light and checked the time: 1.12 a.m. My mind shot back to that night in Tullamore, watching Meehan on top of Eve, the clock radio flipping to 1.13. The boffins say you can’t read the time in your dreams: I hadn’t been dreaming.