It was Jimi Hendrix. When Cooper saw the can of Swan lighter fluid next morning, he knew immediately where he’d seen one before. It featured in one of those classic rock posters. Hendrix setting fire to his white Stratocaster at the Monterey Pop Festival.

Could it have been 1967? Somewhere around that time. The legend said that Hendrix felt upstaged by The Who, because the British group had ended their set by smashing their equipment. During his own last number, the guitarist had grinned at the audience, squirted lighter fuel on his guitar and struck a match, playing the final notes through the flames. It was one of the seminal moments in the history of rock music. Mad, and dangerous.

‘You can buy the hundred millilitre can for about three pounds, but it isn’t stocked everywhere,’ said Fry, when he’d examined the can.

‘That gives us a chance of tracing the shop it was bought from, then.’

‘Yes, it would do, if we had the manpower.’

In the poster Cooper remembered seeing on a friend’s bedroom wall, the can had been clearly visible in the guitarist’s hands. It was just like this one – square-sided and yellow, the same colour as Hendrix’s frilly shirt.

‘Anyway, we’ve got an initial report faxed through from Downie’s people at the FSS lab this morning.’ The neutral tone of Fry’s voice didn’t give away whether it was good news or bad news.

‘What does it say?’

‘I’ll read it for you: “The laboratory received two evidence containers of debris taken from the suspected seat of a fire. A head space sample from each container was subjected to gas chromatograph analysis. The chromatogram shows characteristic peak patterns of a common hydrocarbon fuel, n-Butane.”’

‘Lighter fluid, then.’

‘Right. Specifically, butane lighter fluid. The positive samples were taken from a section of carpet in the Mullens’ sitting room, and from the toy box in the corner near the video. Not much accelerant used – but then, it wouldn’t have needed a lot.’

‘It could have been an accidental spillage, couldn’t it?’ suggested Cooper.

‘Have you tried accidentally spilling lighter fluid, Ben?’

‘I don’t even smoke. I never have.’

‘Well, it comes in an aerosol can like this one, with a pressure valve that fits into the lighter.’

‘OK, I’ve seen it.’

‘The most you can do accidentally is create a bit of mist that makes your fingers feel cold. To spill it, you have to prise the top off the can.’

‘Even so, Diane, one of the Mullens’ kids could have done that.’

‘Maybe. So which of the Mullens was a smoker – Brian or Lindsay?’

‘I don’t know.’

‘The answer is, neither. And why didn’t the SOCOs find a lighter fluid can in the house? They’re metal, so it wouldn’t have been destroyed in the fire.’

‘I don’t know that either.’