‘She was outside Crowther’s flat on Monday night, when I was there—the same person who saw me saw her. She stayed after I’d gone. She knew Seed and Crowther were inside having a cosy evening together, with her picture up on the wall . . .’

‘No one broke in, remember?’

‘Trelease could easily have persuaded Crowther to let her in somehow, or maybe she used the gun from the off, backed Crowther into the flat at gunpoint, down the hall and into the front room where she shot her. She wanted her picture back—perhaps she was jealous, too. If she followed Aidan Seed to London, that suggests he was on her mind.’

‘Maybe you were the one she was following,’ Charlie suggested. ‘Maybe you’re the person she most admires in the whole wide world. No—that really is implausible.’

If she’d been trying to upset him, she didn’t succeed. Usually Simon was easily riled. He only wasn’t when he was in the grip of one of his fixations. Charlie knew the signs: verbal abuse rolled off him like rain off an umbrella. And that occupied look in his eyes, so that you could almost see his brain whirring . . .

‘Trelease killed Crowther and made Seed go with her somewhere, ’ he said. ‘She had a gun with her, must have had a car too. Wherever she took Seed, they went in her car, having first locked the painting in the boot of his, to make sure any suspicion fell on him.’

‘Why must she have had a car?’

‘She couldn’t hold a gun to his head as they walked along the street, could she? If she had a car, she could make him drive, sit behind him and—’

‘I don’t believe this!’

‘You said the picture hooks in Crowther’s gums was a woman’s touch,’ Simon reminded her. ‘Crowther was shot, then someone carefully knocked her teeth out with a hammer and replaced them with picture hooks. Compare that to Seed’s description of strangling Mary Trelease—a killing at close range, her struggling, naked, right next to him, or under him, or on top of him . . .’

‘So now he killed her while they were having sex? Another detail you’ve invented.’

‘. . . Seed feeling his thumbnail pressing into his own flesh as he held his hands closed around her throat . . .’

‘You forget, you’re describing a killing we know didn’t happen.’

‘I think it did,’ said Simon. ‘Aidan Seed killed someone, exactly as he described to me. Not Mary Trelease—someone else.’

‘Then why say it was Trelease?’

‘That’s what we have to find out. The first step’s obvious.’

‘Not to me it isn’t,’ said Charlie.

‘Seed grew up in the Culver Valley on a council estate—that Times article said so. Megson Crescent used to be council-owned. Seed’s in his early forties—let’s assume he didn’t kill anyone before he was eleven . . .’