She extracted the enlarged image to a document and cropped it to show only Naomi with her phone on one page and the flash in the window on another. Along with the original saved photo, she sent each page over to the wireless printer on her desk. She watched from her bed as the printer sputt-sputtered each page, making that gentle steam train rattle as it did. Pip closed her eyes for just a moment, listening to the soft chugging sound.

‘Pips, can I come in and vacuum?’

Pip’s eyes snapped open. She pulled herself up from her slumped position, the whole right side of her body aching from hip to neck.

‘You’re still in bed?’ her mum said, opening the door. ‘It’s half one, lazy. I thought you were already up.’

‘No . . . I,’ Pip said, her throat dry and scratchy, ‘was just tired, not feeling so well. Could you do Josh’s room first?’

Her mum paused and looked at her, her warm eyes staining with worry.

‘You’re not overworking yourself, are you, Pip?’ she said. ‘We’ve talked about this.’

‘No, I promise.’

Her mum closed the door and Pip climbed out of bed, almost knocking her laptop off. She got ready, pulling her dungarees on over a dark green jumper, fighting to get the brush through her hair. She picked up the three photo printouts, placed them in a plastic folder and slid them inside her rucksack. Then she scrolled to the recent calls list in her phone and dialled.

‘Ravi!’

‘What’s up, Sarge?’

‘Meet me outside your house in ten minutes. I’ll be in the car.’

‘OK. What’s on the menu today, more blackmailing? Side order of breaking and enteri–’

‘It’s serious. Be there in ten.’

Sitting in her passenger seat, his head almost touching the roof of the car, Ravi stared down open-mouthed at the printed photo in his hands.

It was a long while before he said anything. They sat in silence, Pip watching as Ravi traced his finger over the fuzzy blue reflection in the far window.

‘Sal never lied to the police,’ he said eventually.

‘No, he didn’t,’ Pip said. ‘I think he left Max’s at twelve fifteen, just like he originally said. It was his friends who lied. I don’t know why, but on that Tuesday they lied and they took away his alibi.’

‘This means he’s innocent, Pip.’ His big round eyes fixed on hers.

‘That’s what we’re here to test, come on.’

She opened her door and stepped out. She’d picked Ravi up and driven him straight here, parking on the grass verge off Wyvil Road, her hazard lights flashing. Ravi closed the car door and followed as Pip started up the road.

‘How are we testing that?’

‘We need to be sure, Ravi, before we accept it as truth,’ she said, making her steps fall in time with his. ‘And the only way to be sure is to do an Andie Bell murder re-enactment. To see, with Sal’s new time of departure from Max’s, whether he would still have had enough time to kill her or not.’

They turned left down Tudor Lane and traipsed all the way to just outside Max Hastings’ sprawling house, where this had all begun five and a half years ago.

Pip pulled out her phone. ‘We should give the pretend prosecution the benefit of the doubt,’ she said. ‘Let’s say that Sal left Max’s just after that photo was taken, at ten minutes past midnight. What time did your dad say Sal got home?’

‘Around twelve fifty,’ he replied.

‘OK. Let’s allow for some misremembering and say it was more like twelve fifty-five. Which means that Sal had forty-five minutes door to door. We have to move fast, Ravi, use the minimum possible time it might have taken to kill her and dispose of her body.’

‘Normal teenagers sit at home and watch TV on a Sunday,’ he said.

‘Right, I’m starting the stopwatch . . . now.’

Pip turned on her heels and marched back up the road the way they’d come, Ravi at her side. Her steps fell somewhere between a fast walk and a slow jog. Eight minutes and forty-seven seconds later, they reached her car and her heart was already pounding. This was the intercept point.

‘OK.’ She turned the key in the ignition and pulled back on to the road. ‘So this is Andie’s car and she has intercepted Sal. Let’s say that she was driving for a faster pick-up time. Now we go to the first quiet spot where the murder theoretically could have taken place.’

She hadn’t been driving long before Ravi pointed.

‘There,’ he said, ‘that’s quiet and secluded. Turn off here.’

Pip pulled off on to the small dirt road, packed in by tall hedgerows. A sign told them that the winding single-track road led down to a farm. Pip stopped the car where a widened passing place was cut into the hedge and said, ‘Now we get out. They didn’t find any blood in the front of the car, just the boot.’

Pip glanced at the ticking stopwatch as Ravi was crossing round the bonnet to meet on her side of the car: 15:29, 15:30 . . .

‘OK,’ she said. ‘Let’s say that right now they are arguing. It’s starting to get heated. Could have been about Andie selling drugs or about this secret older guy. Sal is upset, Andie’s shouting back.’ Pip hummed tunelessly, rolling her hands to fill the time of the imaginary scene. ‘And right about now, maybe Sal finds a rock on the road, or something heavy from Andie’s car. Maybe no weapon at all. Let’s give him at least forty seconds to kill her.’