Fry cursed to herself as she left Jed Skinner and got back in the car. Male friends, what a waste of time. To learn anything about the state of the Mullens’ marriage, she needed to talk to Lindsay’s mother. But she didn’t give much for her chances of getting information out of Mrs Lowther right now.

With a frown, Fry turned to her notes from the interviews with Brian Mullen, seeking the smallest clue. After a few minutes, she picked up the phone and called Cooper.

When his phone rang, Cooper was standing by the lid of a shaft into the hillside that had been sealed by a steel grille. A bush rustled, showering drops of moisture, and a small, grey shape slipped along a branch, stopping to pull off the berries.

‘Ben, what are these illuminations that Brian Mullen mentioned?’

‘Illuminations?’

‘I’m sure he said they were in Matlock Bath. The only illuminations I know of are in Blackpool.’

‘Well, they’re not quite the same. In Matlock Bath, there are some lights along the promenade and across the river, but when people talk about the illuminations they mean the parade of boats.’

‘Boats?’

‘They create designs out of lights and mount them on rowing boats. Then they parade up the river – when it’s dark, of course. So what you see isn’t the boat but something like, say . . . an illuminated London bus floating on the water. There’s other stuff, too – fireworks, entertainment. You can see it all from the pleasure grounds in Derwent Gardens.’

‘OK. So when does this happen?’

‘September and October, but only at weekends. They call them Venetian Nights. I don’t know why, it must be something to do with the boats. But they always attract big crowds. Why, what are you thinking?’

‘Brian Mullen. When I interviewed him in hospital, he said that he and Lindsay had promised to take Luanne and the other children to see the illuminations in Matlock Bath. It was supposed to be a special treat.’

‘Yes, but surely he’d have more sense than to . . .’

Cooper stopped speaking, and Fry laughed. ‘What was it you were saying earlier, Ben? About people acting in an irrational way?’

‘Emotions interfere with rational behaviour.’

‘That was it.’

‘Diane, why were you so sure about Mr Mullen being involved in the fire?’

‘He never seemed particularly grief stricken to me. Some of those people leaving flowers outside the house looked more upset than Brian Mullen did.’