He smiled. ‘That day at her flat, while Rebus was watching me and you were working on her computer … he told me something, said I was the only suspect.’

‘You thought you’d try throwing us off the scent?’

‘It was just supposed to be that one message … but when you replied, I couldn’t resist. I was as hooked as you were, Siobhan. The game had us both.’ His eyes sparkled. ‘Isn’t that something?’

He seemed to expect an answer, so she nodded slowly. ‘Are you thinking of killing me, David?’

He shook his head briskly, irritated by the assumption. ‘You know the answer to that,’ he spat. ‘You wouldn’t have come otherwise.’ He walked over to a low headstone and rested against it. ‘Maybe none of it would have happened,’ he said, ‘without the Professor.’

Siobhan thought she must have misheard. ‘Which one?’

‘Donald Devlin. First time he saw me afterwards, he guessed I’d done it. That’s why he came up with that story, someone loitering outside. He was trying to protect me.’

‘Why would he do that, David?’ It felt strange using his name. She wanted to call him Quizmaster.

‘Because of everything we talked about … committing murder, getting away with it.’

‘Professor Devlin?’

He looked at her. ‘Oh yes, he’s killed too, you know. Old bugger as good as said so, daring me to be like him … maybe he was just too good a teacher, eh?’ He ran his hands over the headstone. ‘We had these long talks on the stairwell. He wanted to know all about me, my early days, the angry days. I went to his flat once. He showed me these cuttings … people who’d disappeared or drowned. There was even one about a German student …’

‘That’s where you got the idea?’

‘Maybe.’ He shrugged. ‘Who knows where ideas come from?’ He paused. ‘I helped her, you know. She was dead impressed, all those clues … pulling her hair out until I came along …’ He laughed. ‘Flip was never much good with computers. I gave her the name Flipside, then sent the first clue.’

‘You turned up at the flat, told her you’d solved Hellbank …’

Costello nodded, remembering. ‘She wasn’t going to go with me until I promised to drop her off afterwards … She’d just kicked me out again – final this time, she’d piled my clothes on a chair – and after Hellbank she was heading off for a drink with all those bloody friends of hers …’ He screwed his eyes shut for a moment, then opened them and blinked, turning his head to face Siobhan. ‘Once you’re there, it’s hard to go back …’ He shrugged.

‘There never was a Stricture?’

He shook his head slowly. ‘That clue was all for you, Siobhan …’