Becks hit the button a few times until the time stamp in the bottom corner started whizzing forward, and Judith could see that the choirmaster and choir were in position the whole time. Then, as the clock rushed up to 8 p.m., the boys all suddenly left, and plenty of the adults left their positions, chatting to each other or heading off for a few minutes. But Elliot stayed where he was.

By 8.07 p.m., the adults were all back in the choir stalls, and the rehearsal carried on. And still Elliot hadn’t moved. Not once. Nor did he pluck out his mobile to make a call or check it in any way. He just sat on his own in quiet contemplation.

‘He didn’t go anywhere,’ Judith said, disappointed.

‘Well, that’s good news, isn’t it?’ Becks said.

Becks kept the footage whizzing forward, and the two women saw that Elliot didn’t leave his position until shortly after 9 p.m., when the rehearsal began to break up for everyone.

Becks took her finger from the mouse, and the footage resumed playing at normal speed.

‘So there we are,’ Becks said. ‘Elliot was there from seven to nine. He can’t be your murderer. Thank heavens.’

‘I suppose not.’

Judith felt cheated. This wasn’t the answer she’d been looking for, and as she watched Elliot chatting easily with three other men as they left the rehearsal, she had to admit that he wasn’t behaving like someone who’d been involved in a murder, either. He seemed relaxed and in good spirits.

‘Hold on,’ Judith said, indicating the monitor, ‘what was that?’

On the screen, Elliot had just passed out of shot with his friends.

‘What was what?’

‘Can you rewind to show Elliot leaving?’

Becks rewound the footage, Elliot and his friends walking backwards into the frame.

‘Can you replay it from there?’ Judith asked.

Becks pressed play and Elliot and his friends started to walk down the aisle again.

This time, both Judith and Becks saw it.

Just before he disappeared from view, Elliot looked directly into the lens of the camera. But almost as soon as he’d looked, he turned and clapped one of his friends on the back, the men all leaving, chatting happily together.

‘He looked at the camera,’ Becks said.

‘He did, didn’t he? He knew it was there.’

‘I suppose he’s had years to notice it. Maybe it’s a coincidence?’

‘I don’t think so. You saw it too, didn’t you? That look on his face.’

Becks bit her lip. It was true. She’d seen it as well.

‘It’s like he knew someone would watch the footage. To check up on his alibi. And for a split second his ego took over and he couldn’t help himself. He had to look at the camera.’

‘But how would you describe that look on his face?’