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.’

They waited.

‘So now Andie’s dead.’ Pip pointed down at the gravel road. ‘He opens the boot –’ Pip opened her boot – ‘and he picks her up.’ She bent down and held out her arms, taking enough time to lift the invisible body. ‘He puts her inside the boot where her blood was found.’ Pip laid her arms down on the carpeted boot floor and stepped back to shut it.

‘Now back in the car,’ Ravi said.

Pip checked the timer: 20:02, 20:03 . . . She put the car in reverse and swung back out on to the main road.

‘Sal’s driving now,’ she said. ‘His fingerprints get on the steering wheel and around the dashboard. He’d be thinking of how to dispose of her body. The closest possible forest-y area is Lodge Wood. So, maybe he’d come off Wyvil Road here,’ she said, turning, the woods appearing on their left.

‘But he would have needed to find a place to get the car up close to the woods,’ said Ravi.

They chased the woods for several minutes searching for such a place, until the road grew dark under a tunnel of trees pressing in on either side.

‘There.’ They spotted one together. Pip indicated and pulled off on to the grassy verge that bordered the forest.