‘Didn’t the IRA use saltpetre in their bomb-making operations at one time?’ said Fry. ‘I had a feeling it had been put on a restricted list, not available for sale to the general public.’

Hitchens laughed. ‘You can make bombs from sugar and fertilizer, so why would anyone worry about saltpetre?’

‘Also, it’s been implicated in having carcinogenic properties.’

‘I don’t think anyone was worrying about getting cancer or making bombs,’ said Cooper.

‘Oh?’

‘This other stuff in the kitchen. Did it include sesame seeds, by any chance?’

‘Yes, it did,’ said Hitchens. ‘How on earth did you know that, Ben?’

He’d have to ask Amy for the exact wording, but Cooper thought he could remember the recipe pretty well.

Squeeze out the blood. Embalm it in a shroud and steep it in a solution of saltpetre, salt and pepper for two weeks, then dry in the sun. The candles are made from a hanged man’s fat, wax and Lapland sesame.

‘I’d like to make a prediction,’ he said. ‘I predict that if we find another body at Pity Wood Farm, it will be missing a hand.’

Within a few minutes, and thanks to the internet, Cooper knew how to make potassium nitrate himself. The practical part was simple. You could either dissolve solid fertilizer in boiling water, or boil down a liquid fertilizer until crystals started to form. When the solution had cooled to room temperature, it was placed in a fridge. The white crystalline precipitate was mainly KNO3. Garden products tended to contain ammonium nitrate, too, which contaminated the KNO3.

‘We should have picked this up earlier,’ said Hitchens. ‘Potassium nitrate can cause eye and skin irritations. Breathing it in can irritate the nose and throat, causing sneezing and coughing. High levels can interfere with the ability of the blood to carry oxygen, causing headaches, fatigue, dizziness and a blue colour to the skin and lips. Even higher levels can cause trouble breathing, collapse and death. Long term, potassium nitrate may affect the kidneys and cause anaemia. Chronic long-term health effects can occur some time after exposure and can last for months or years.’

‘Is this really something you ought to keep in your fridge?’ asked Murfin.

‘No, Gavin.’

‘And what was it that Derek Sutton died from, did you say?’

‘Heart failure.’

‘That appears on so many death certificates. It’s what doctors write in when they can’t see any other cause of death but don’t want to put the family through the ordeal of a postmortem.’

‘Obviously, no one would have suspected potassium nitrate poisoning at the time, so there wouldn’t have been any toxicology done, even if there had been a PM,’ said Fry.

‘Well, it’s academic, since there wasn’t a postmortem,’ said Hitchens. ‘Derek Sutton was signed off, certificated and cremated within a week.’

‘It would be cremation, of course. So no chance of getting an exhumation order.’