And that was the last thing she got out of him. Mullen remained silent and sullen all the way to Darley Dale. Now and then, he glanced anxiously out of the window at people passing on the pavement, and once he gave a sudden start when a car pulled alongside them at the traffic lights. Fry had no idea what he was so worried about, but she didn’t think it could be her driving.

Finally, she dropped him off outside the Lowthers’ gate, and he thanked her ungraciously. Fry watched as Moira Lowther came out of the bungalow and hugged Brian near the chiminea. That was just like Mrs Lowther. Very keen on hugging people.

Instead of heading straight back to West Street, Fry decided to call at the mortuary to collect the pathologist’s report. Mrs van Doon was in her office and greeted her visitor personally.

‘Yes, all three of your victims had carboxyhaemoglobin levels over fifty per cent – which is sufficient in itself to account for death. Levels of up to ten per cent would be normal, anything more indicates inhalation of carbon monoxide. I believe there might also have been hydrogen cyanide present in the fumes from the fire. That’s a particularly potent toxin, with a rapid action.’

‘Where would that come from?’ asked Fry.

‘Hydrogen cyanide? It’s produced by materials which contain nitrogen – wool, silk. And polyurethane.’

‘Polyurethane, as in furniture foam?’

‘Yes, possibly.’

‘Which of those would explain why three people failed to escape from a house fire when they were woken by the smoke?’

‘Hypoxia resulting from high carbon monoxide levels. At the levels present in these individuals, I’d say they would certainly have been feeling ill and disorientated at the least. They might even have been unconscious.’

‘But we’re sure they died as a result of the fire?’

‘Yes. Soot particles can stain the mouth and pharynx of a person who’s already dead, but if soot is present beyond the vocal cords it means the victim was alive during the fire.’

The pathologist produced a photograph. It was meaningless to Fry, which was probably for the best.

‘All three post-mortems showed evidence of soot in the airways. The two children also had it in the oesophagus and stomach. Those black streaks on the mucus of the trachea indicate that your victims were alive at the start of the fire. Alive, but not necessarily conscious.’

The waitress from the Riber Tea Rooms had arrived at West Street to be guided through the process of producing some e-fit pictures of the people Rose Shepherd had met in Matlock Bath the day before she died. She did her best, but she wasn’t sure about the details.

‘Thank you for coming in anyway, Tina,’ Murfin was saying as Fry walked in.

‘It’s not much use, is it?’

‘You did your best.’

‘I’m really sorry. I wanted to help.’