His dad pointed towards the industrial estate. ‘They think he kept a spare car here. One not linked to him, that he could use to flee if he needed it.’

He looked out of the front window at the four sprawling, derelict buildings. On one side was a scruffy field; on the other was run-down housing estate. Behind was the slick, oily canal.

James stared at it.

Best had talked to him once about canals, when he was tutoring him. What was it he’d said? Something about what he’d do if he won the lottery?

He grabbed his dad’s elbow.

‘Dad,’ he said. ‘I know where she is.’

Wynne

DI Wynne stood by the gates of the industrial estate. She had met James Cooper years ago and it was hard to believe this was the same person. He was thin, his hair patchy, his eyes sunken in a pallid, slack face.

‘So,’ she said. ‘You have an idea where your sister may be?’

James shuffled from foot to foot.

‘Maybe,’ he said. ‘Sort of.’

‘Talk us through it,’ Wynne replied.

‘It’s the canal,’ he said. ‘He brought her here because there’s a canal. He didn’t switch to a car, he switched to a boat.’

Wynne folded her arms ‘You think he kept a boat here?’

‘Yes.’ James looked at her. ‘When he was my tutor we were doing probability. He used the lottery as an example. He asked if I thought there was more chance of a set of random numbers coming up than a sequence of numbers – say one to six. I thought it was random but he showed me why it was mathematically the same. Then I asked him what he would do if he won the lottery. He told me he would get a canal boat and live on it. I remember him saying how you could go anywhere you wanted on the canals and no one would know. They go over the whole country …’

‘Did he have a boat?’ Wynne said. ‘Did he ever mention that?’

‘No,’ James said. ‘But I’ve not seen him for years. He could easily have bought one.’

‘And if he had,’ Wynne said, ‘using it to conceal them would make sense. He’d know we’d be looking for a car.’

James Cooper turned to his dad. ‘Them?’ he said. ‘He has more than one prisoner?’

DI Wynne glanced at the PC. ‘Let’s go and look for evidence of a boat,’ she said. She turned to Martin Cooper. ‘I think you might need to talk to your son for a few minutes.’