And he did.

‘She asked you to go instead of her? You were the one confirmed that day?’

‘Yes.’

It all came out in a rush now.

‘We arranged to meet at the den, where Mary changed back into the school clothes she’d brought from home. I put on the dress and veil and went to the chapel. When I got back, Mary wasn’t there waiting like we arranged. I thought she was probably hiding in case someone spotted her and realized she hadn’t gone to the ceremony. I changed back into my own clothes and left the dress, the veil and shoes in the plastic bag as she’d asked.’

She halted there, her face stricken with remorse.

‘It was my fault she died. If I’d said no that day, she wouldn’t have been there. He wouldn’t have got her. She would have been safe.’

McNab waited, although his brain was working overtime on what she’d just told him. For the confirmation clothes to be buried with Mary, her killer must have been to the den, before or after he’d taken Mary.

‘We found a blue jumper in a box in your loft,’ McNab said quietly. ‘Was that Mary’s?’

Karen seemed startled by the question, then said guiltily, ‘After everyone said she was missing, I ran back to the den to look for her. The dress and the bag were gone. I wondered if she was hiding somewhere in the woods. I went looking, shouting her name. That’s when I found the jumper.’

She hesitated. ‘I should have given it to the police, I know, but I hoped she’d come back. I was a wee girl who’d lost her best friend and it was the only thing I had of hers.’

‘Did you ever wash the jumper?’ McNab said.

‘Oh no. My mother never knew I had it. I kept it hidden. Like the diary.’

If Mary was wearing the jumper when she’d met her assailant, then evidence of that contact could still be on it. In 1975, forensic science, in its infancy, would likely have gained little from it. And considering other evidence had disappeared during the years in between, maybe the jumper would have done the same.

If the confirmation outfit had remained in the bag, then chances were it had never been handled by the killer, which was why they’d only found Mary and Karen’s DNA on it. As for the jumper . . . McNab allowed himself a small glimmer of hope.

And now to the tricky question.

‘Who do you think sexually assaulted Mary?’ McNab tried, knowing this might prove the key to everything.

Karen didn’t or couldn’t meet McNab’s eye. ‘I didn’t ask. I didn’t want to know. All I wanted was to wear that dress.’

‘But you think you might know now?’ McNab tried.

‘I don’t know,’ she repeated.

‘But you suspect?’ McNab tried again.

Her response, when it came, didn’t answer his question.

‘My dad told me I had to forget everything that happened that day. Everyone in the street suspected everyone else. My dad said we had to protect the innocent and not listen to gossip or repeat it. My mum was terrified that whoever had taken Mary would come back for me.’