‘You know, I think there was a change in her,’ the professor said. ‘She seemed distracted on my last visit. Somehow disengaged.’

‘Could you go through your movements again, for the afternoon that Margaret was killed?’ Joe couldn’t imagine this man as a murderer, but Vera had found discrepancies in his evidence and she’d slaughter him if he didn’t check. ‘You were out in the boat with Malcolm Kerr?’

‘That’s right. Part of my regular fieldwork off Coquet Island.’

‘And what time did you get back to Mardle?’

‘I told that young woman who came to the laboratory to talk to me.’ There was no resentment in his voice, but a kind of resignation. ‘It was about three o’clock.’

‘And what did you do then?’ This was the important question. Craggs had told Holly that he’d driven straight home, but Enderby claimed that they’d met up in Harbour Street later in the day.

‘I went to the Dove Laboratory in Cullercoats. I had equipment to drop off there.’

‘And then?’ Joe leaned forward across the table.

‘Then I went back to Mardle. It was a nuisance. It was snowing heavily and I wanted to get home. But I’d left my briefcase in Malcolm’s yard – one of those senior moments that seem to happen more frequently these days – and I had an important phone call to make the following morning. I knew I’d need the papers. I have a key to the yard and to Malcolm’s shed, so I didn’t need to disturb him. And in Harbour Street I bumped into George. He seemed so miserable that I couldn’t leave him there alone. We had one drink in the pub. I thought if the snow was really bad I could always stay the night at Kate’s. In the end it seemed to have cleared a bit, so I drove home.’

The words came easily. Too easily? Joe wondered if they might have been rehearsed. ‘And you invited Mr Enderby to spend a night here with you?’

‘Not that night, but two days later, yes. He’d got himself into a state. He’s obviously told you that his wife has left him. He’d run away to Harbour Street, still pretending that he was working. Because he couldn’t face telling Kate what had happened, I invited him here on the day that he claimed to be in Scotland.’

Mary arrived with coffee and melted discreetly away.

‘How did he seem when he was here?’ Joe asked.

‘Distraught. He drank too much of my whisky and became incoherent. We knew that Margaret was dead by then, of course. Her death seemed to have upset him almost more than his wife’s leaving him. We spent a lot of time talking about her.’ The professor drank coffee, leaning back in his chair.