Downie smiled, a man who enjoyed the small details of his job. ‘Old houses are the worst. A lot of them are like bonfires waiting for the first match. Wooden floors, wooden beams and window frames. With a bit of breeze blowing through, you can get a fire going that’s hot enough to melt the fillings in your teeth. Of course, it isn’t actually the wood that burns but the gases released from it by heat. The solid material disintegrates, and you’re left with a pile of ashes.’

‘But the victims died upstairs,’ said Fry. ‘Smoke inhalation.’

‘Oh, yes. Absolutely lethal. If you get a lungful of smoke from a house fire, you’re in trouble.’

‘I can’t understand why the victims never even made it to the stairs.’

‘Look, it goes like this . . .’ Downie demonstrated by closing his eyes and clutching his throat. ‘You’ve taken a breath and you can’t open your eyes because as soon as you do they water. You take another breath and the irritants hit the back of your throat. You retch and take an even deeper breath – it’s a natural, involuntary reaction. It fills your lungs with toxic fumes. That disorientates you, makes you dizzy, and puts you down on the floor. While you’re incapacitated, the toxicity takes over.’

To Fry’s amazement, he lay down on the floor and demonstrated what it was like to be dead. She’d never seen anybody look less dead in her life. But if there had been a weapon handy, she might have felt tempted to help him achieve authenticity.

Then he opened his eyes and looked up. ‘We used to say you had seven minutes to get out of a burning building. Now, with all the materials we’ve put inside them, it’s more like three minutes.’

‘And that’s why we advise people to install smoke alarms.’

‘Ah, yes. The smoke alarm. Pity about that.’

‘It was functioning, wasn’t it?’

‘After a fashion.’

‘Mr Mullen says he tested it regularly.’

‘No doubt he did. But, like most people, all he was doing was pressing the button. That proves the sound works, and the batteries aren’t dead. It doesn’t tell you whether the detector is functioning.’

‘What are you saying?’

‘I took a look at the smoke alarm earlier. And I’d say your Mr Mullen never bothered reading the manufacturer’s instructions. He should have vacuumed around the detector regularly to prevent the build-up of dust. Apart from battery failure, there’s nothing worse than accumulated debris for interfering with an alarm. This one hadn’t been cleaned for a long time. There was even a thin layer of cement and plaster particles, so I’m guessing the family had building work done in the kitchen at some time.’

‘Yes, they had new units put in about six months ago, and an extractor fan installed.’