I found one single reference to a woman on the street at the relevant time. The statement read: ‘At about five forty p.m., Charles Crosby, of 74 Vardens Road was cycling home from work along Sangora Road when he saw two women coming down the steps of either 21 or 23. The second woman carried a black gym bag and may have been black.’

I dug out testimony from the residents of number 23 – the house next door to the murder scene. Angela Adeyemi, an IC3, or African female, said she left the house with a friend at about five thirty-five that evening to attend a gym on Clapham High Street. This surely torpedoed any hopes that Crosby had seen Karen and her accomplice fleeing the scene.

Just after eleven, Shep almost ran into the office. ‘I’ve just had my nuts blowtorched for two hours,’ he said, ‘we’ve got to make this collar happen.’

I handed him the note detailing what Crosby saw. Then, with an apologetic grimace, I passed over the statement from Ms Adeyemi.

Shep read both, gave them back and said: ‘Get Crosby in, as soon as possible.’

As he walked off, I gave Mick a quizzical look.

‘Desperate times …’ he said.

I rang Crosby’s home phone number. His haughty wife delighted in informing me that he couldn’t possibly make it to the station until after four p.m.

Just before lunchtime, DS Barratt returned with his forensic ferrets, fresh from their forage of the Foster family home. He sported the smug gait of a man with sterling news.

First, he produced half a dozen true crime books, retrieved from Karen’s old bedroom: these included the bestselling Murder Scene Secrets, by Professor Laurence C. Richards BSc, MSc, FRSA.

‘This is how Karen learned to thoroughly and comprehensively trash our crime scene,’ said Barratt.

‘Can you believe Richards is allowed to make money giving away our techniques?’ barked Shep.

Barratt said they were wrapping up their search when he spied a mop in the garage.

‘And look what I found buried in the mop’s handle,’ he said, pulling out a metal ruler which had been pared down to a point at one end. ‘This end is potentially lethal,’ he said, ‘and it measures five inches. As we know, that’s about the length of the blade used to kill Marion. Karen’s dad, Terry, is a window cleaner, she and her sisters often help him at the weekends. He told us he uses this ruler to scrape dirt out of awkward corners.’

I couldn’t help thinking a ruler as a murder weapon looked a little desperate.

‘When I asked him about the day of the murder, well, that seemed to be his most awkward corner yet.

‘He told us that he got home at his usual time, three p.m. He said he left the mop in its usual place, standing in the corner of the garage. We asked him if he remembered this metal ruler being there the day after the murder. He got really agitated and, get this, said he couldn’t remember. I pressed him and he said it again, he couldn’t remember.’

Shep decided to do our thinking for us: ‘Karen wouldn’t have had access to potential murder weapons. She could have popped home on the afternoon of the murder, when she was supposedly shopping in Blackheath, picked that up before driving to Marion’s.

‘After killing her, Karen had to get straight back to the Pines, because Bethan Trott and her sister Laura were her alibis. She must have brought the weapon back with her and stashed it somewhere before going to see Peter to do the fish. Then, at a later date, she would have slipped the weapon back into the mop.’

It was plausible, if a little stretched. Without forensic evidence, I felt a jury would never buy it,

Shep went on: ‘I trust you took it to the lab?’

‘Got it checked out, right away. Of course it’s clean,’ said Barratt, ‘but at least you can wave it in front of Karen, see how she reacts. Judging by Terry’s response, this is our murder weapon and they know it.’

Shep told him to take the blade to the pathologist: ‘See if he’ll confirm that it could have been the murder weapon, in terms of shape, size, sharpness.’

‘There’s something else,’ said Barratt, relishing his moment in the sun and wringing it out for all it was worth. ‘Karen still gets some of her post sent to her family home. When I looked on top of the fridge, I found her bank statement from July. It seems that when she was shopping in Blackheath with her sister Laura on the afternoon of 1st July, at four ten p.m. her cash card was withdrawing ten pounds from Lambeth High Street – just up the road from the Pines.’

Shep grabbed and scoured the statement: ‘So much for Karen’s rock-solid alibi. The only people backing her story now are her sister Laura and the woman they watched TV with from five thirty to six that day. What’s her name?’

‘Bethan Trott,’ someone said.

‘Get her in, Barratt, as soon as you can.’

Charles Crosby turned up bang on four, bang on stereotype. Late forties, cowlick fair hair, square face, strong chin, pinched pink cheeks, chunky knitted pullover, big tits, big arse, mustard corduroys: ‘A good cove,’ was how any judge would view him. I’d always found it fascinating how the two most pronounced social stereotypes in Britain are the richest and the poorest: the Toffs and the Chavs. Maybe they’re not as different from each other as they think.

Unlike my colleagues, I held no inferiority-based grudge against posh English people. To me, they seemed very polite and very sexually repressed – characteristics I could readily relate to.

As instructed by Shep, I took Crosby into his office and left them to chat. About fifteen minutes later, Shep called me in.

‘Mr Crosby has kindly agreed to give us his statement. Can you write it down for him please?’ The media studies lecturer wasted no time getting to the important bit: ‘At about five forty-five p.m., I was cycling down Sangora Road on my way home from work when I saw a man and a woman coming down the steps of number 21. I didn’t get a close look but the woman was aged between twenty and thirty, had long dark hair and wore a red top. Both she and the man were white Europeans. The woman carried a black gym bag.’

‘Thank you, Mr Crosby,’ Shep cut in abruptly, ‘we really have taken up enough of your very valuable time.’

‘Thank you, Superintendent,’ said Crosby, ‘I really do appreciate you putting me forward for the remuneration.’

‘Think nothing of it,’ laughed Shep, shaking his hand, ‘that’s what it’s there for.’

As soon as Crosby was out of earshot, I asked: ‘What was that about?’

‘He’d heard that Marion’s family have put up a £20,000 reward for information leading to her killer. I was just assuring him that his evidence may well put him in the running for it. Anything to keep a key witness happy.’

I struggled to understand how anyone with key information about a tragic murder could even think about money. Maybe it was a class thing.

‘Didn’t he originally say that he saw two women, and that one of them may have been black?’

‘Well, he seems to have cleared it up in his own mind,’ said Shep, as a knock sounded on his office door.

‘Come in,’ he called.

I wanted Karen Foster banged up, not fitted up.

I reminded Shep: ‘What about the black woman who lives at 23? Shouldn’t we check if it could have been her and her friend heading to the gym?’

‘Mr Crosby has made his statement,’ said Shep, absently, signing for two packages. I noticed that one of the parcels came from Woolwich CID, the second from the Agfa video transfer company.

As the courier shut the office door behind him, Shep turned to me and said: ‘If we can turn over Bethan Trott, we’ve got a case.’

I remember Fintan’s words that afternoon at the Feathers: When he gets a sniff of a collar, he goes proper psycho. Like a bloodhound.

He handed me the first package: ‘This is a preliminary case report into the Bisset murders. Take a very thorough read. See if you can find more reasons why the nutter who did this didn’t kill Marion. Report to me by lunchtime tomorrow.’