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 …’

‘I don’t know why you kept going back to her, David, or what you thought the game would prove, but one thing I do know: you never loved her. What you wanted was to control her.’ She nodded at the truth of this.

‘Some people like to be controlled, Siobhan.’ His eyes were staring into hers. ‘Don’t you?’

She thought for a moment … or tried to think. Opened her mouth and was about to speak, but a noise interrupted. He snapped his head round: two men approaching. And two more fifty yards beyond them. He turned slowly back to Siobhan.

‘I’m disappointed in you.’

She was shaking her head. ‘Not my doing.’

He leaped from the headstone, hurtled towards the wall, his hands reaching the top, feet scrabbling for purchase. The detectives were running now, one yelling, ‘Stop him!’ Siobhan just watched, rooted to the spot. Quizmaster … she’d given him her word … One of his feet had found a half-inch of ledge, pushing up …