The case rewound before my eyes, to a soundtrack of the door of 21 Sangora Road slamming shut, over and over. ‘Oh my God,’ I said out loud.

I turned and chased Shep, already galloping towards the kitchenette.

‘We’ve got to ask her about her trainers, Guv,’ I said. He looked at me with withering contempt.

‘Look, Lynch, our case is falling apart in there …’

‘Please, Guv, I’m serious. Just get them to ask her where she got them. Please, you’ve got to trust me on this.’

‘Jesus, Lynch,’ sighed Shep, shaking his head, ‘this better be good.’

Back in the interview suite, before switching on the tape recorder, Good cop Mick cracked a bashful smile and said: ‘Laura, can I ask you something before we start, though it’s a little embarrassing?’

She looked sideways at her solicitor, then back to Mick.

‘It’s just that we’ve got a very fashion-conscious WPC in the team who’s really taken a shine to your trainers. She just wanted to know where you got them from.’

Laura turned again to her solicitor, her frown flipped, clearly dying to elaborate. Her solicitor shrugged as if to say: ‘No harm in it, I suppose.’

‘They’re Nike Air Huaraches,’ she announced loftily. ‘My uncle sent them over from the States for my birthday last month. They’re not even on sale in the UK yet.’

I told Shep I’d be back in a few minutes and ran into a nearby office. I called Fintan for one reason: he had an extensive cuttings library at his behest. He didn’t answer, so I paged him. He knew I’d only do this in an emergency. He called back right away. I told him to find out all he could about Nike Huarache shoes and to let me know as soon as possible. He didn’t dare ask why or object.

He got back to me in record time.

I learned that the Nike Huarache trainer was the brainchild of Tinker Hatfield, also the designer of Air Jordans and the Air Max. It was inspired by his water-skiing boots, and has a sock-like lining which they called Dynamic Fit.

‘Is any of this relevant?’ asked Fintan.

‘Just keep talking.’

Because of their unusual design, sales of the Huarache shoe hadn’t taken off. Last year, Nike didn’t get enough pre-orders to go into production. But then, a few months ago in April, some marketing guru decided to sell them, guerrilla-style, at the New York Marathon. Suddenly demand soared. Last month, Nike had re-launched the Huarache in the US, but they weren’t scheduled to be sold in the UK until October.

I raced back to Shep and told him the news.

‘There can’t be more than a handful of Huarache shoes in the whole of the UK,’ I said.

‘But forensics combed the scene for footprints. If they found prints from a shoe that rare, they would have flagged it up,’ said Shep.

‘Yes but I want them to check the doors.’

‘For shoeprints?’

‘Yes. You’ll see why, next time they take a break,’ I said, my growing conviction somehow eclipsing my inner terror at making a total arse of myself, yet again.

By now, Laura had reverted to her ‘no comment’ wall stare. Good cop Mick tried reason. Bad cop Colin attempted terror. He managed to scare the shit out of everyone, except Laura.

I stood and walked to the corner of the two-way to get a good look at those trainers. Beside the solicitor’s brogues, they looked tiny.

‘Get them to ask her what size shoe she is,’ I said.

Shep shuffled in his seat, irritated and reluctant.

‘Please, Guv, it’s just one more question.’

He paged Mick and met him at the door to the suite. I could see Shep having to work really hard to convince him. As Mick shut the door, he turned to the two-way, shook his head and mouthed, ‘wanker’.

After a time, Laura’s solicitor asked Mick if his client could take a bathroom break.

‘Of course,’ said Mick, terminating the interview and switching off the tape recorder.

As they all got to their feet, Mick smiled at Laura and said: ‘Before you go, our foot fetishist was wondering what size shoe you take.’

‘These are a size three,’ she smiled, ‘and even that’s a bit big for me. I have to wear thick socks.’

I ushered Shep to the corridor: ‘I want you to watch them go through the security door, Guv, really closely.’

After ten seconds, I almost had to shove his sceptical arse out to the corridor ahead of me. I hoped to God Laura would do the same as she did before. Otherwise, my theory would never fly and my career might crash land before it even had the chance to take off.

We watched the trio of WPC, Laura and lawyer walk towards that security door. As before, the WPC pressed the green release button, pulled the security door towards her and walked through first. Laura took the weight of the door with her arm, then turned her back against it, once again inviting her solicitor through next. As he waddled closer, I willed her to use her foot again.

‘Use your foot, Laura,’ I mouthed at her head, ‘use your foot.’

As the solicitor got within touching distance, her foot went up, her trendy green sole planting itself on the door and pushing it back, right up against the wall.

‘You see that, Guv?’ I said.

‘See what?’ he said.

‘What she did with her foot.’

He nodded.

‘The door to Marion’s flat is spring-loaded, like that one.’

‘Shit!’ He was with me.

‘The evening of the murder, when they waited for Marion at the bottom of the steps of number 21, Karen would have held the gym bag containing the change of clothes and the weapon. After all, she was the older sister and the one with the beef. To ensure Marion didn’t notice or question the bag, she stood behind Laura.’

Shep nodded, happy for once to ride pillion.

‘Marion would have led the way up the garden steps and through the front door, followed by Laura, then Karen. Marion picked up her post. She already had a handbag and a coat over her arm. When she pulled open that spring-loaded door to the flat, she had no spare hand to hold it open for Laura, who wouldn’t have been expecting the weight of the door.’

I could almost see celestial light passing across Shep’s rapt face.

‘Marion unlocked and opened the flat door towards her, walked through. Laura came next. She felt the weight of the door, turned her back against it to let Karen through ahead of her. But to open it wide enough to let fatso past, she needed to use her foot.’

I headed off Shep’s next question before he had the chance to ask it.

‘We know they probably destroyed their blood-soaked clothes, but there’s no way Laura would get rid of a pair of two-hundred-quid designer trainers.’

‘Two hundred quid? You’re joking.’

Shep flew up the corridor, me in hot pursuit.

He explained the scenario to the forensics officer.

‘What are the chances that a shoe print could still be on that door, after seven weeks?’

‘Well, it’s a flat surface. It’s not like a door handle that gets touched every two minutes. I’d say if she left a shoe print on that door, part of it should still be there.’

‘If the print is there, can we prove that it belongs specifically to her shoe?’ I asked.

‘They’re better than fingerprints actually, because they usually leave a more specific pattern. We can match it for design, size, even how worn the sole is. If we find a trace, then you’ve hit the jackpot.’

Chapter 38

Clapham Police Station, South London

Sunday, August 18, 1991; 13:00

Shep sent a team to formally arrest Karen Foster and bring her back to the station. We stood together at the custody desk as Laura was led out of her cell, minus the trendy trainers that had sealed her fate. She looked composed, impenetrable. As she was being processed, I heard the security door beep. I peered around the corner to see two officers leading Karen in. She looked flustered, scared, a busted flush.