Mouldy, fusty, stale and mildewed. Those were the familiar smells of the hut. If the weather was warm, he could scent an underlying odour of oil or grease, saturated into the concrete from whatever had gone on in here. That smell must have lasted decades. Fifty, sixty years? The buildings must be from about that time. They were so old-fashioned, so last century. It was hard for him to imagine what anyone might have done up here, stuck on an empty hillside in Derbyshire.

Now and then, he smoked a spliff up here at the huts, but he couldn’t afford that now. Instead, he plugged in the earphones of his iPod and selected some Coldplay. A feeling of peace settled over him as he listened to Chris Martin’s plaintive vocals coming in on ‘A Rush of Blood to the Head’. He could forget everything else for a while once the music was playing.

Today, though, Sean knew something was different. He tugged out the earphones and stayed completely still, his eyes tightly closed, his ears straining for a sound. The scurrying of a mouse under the floorboards, maybe, or a bird scratching a nest in the roof. But there was no sound.

He squinted at the dust swirling slowly around him, disturbed only by the current of his own breath. The room looked the same as always. Nothing had been moved or disturbed since his last visit. It was always a worry that someone else would find the derelict buildings – another vagrant sleeping rough, a couple of kids finding a place to have sex or take drugs. Or, worst of all, the owner coming to check on his property, or a builder with a plan to demolish it.

Sean closed his eyes, trying to recapture the moment that had been lost. But he finally had to acknowledge that something really was different today. It wasn’t a sound, or anything that looked out of place. It was in the very quality of the mustiness, an underlying odour that was too sweet to be oil or grease. He couldn’t deny the message that was hitting his nostrils.

The difference was in the smell.

IT HAD BEEN raining for six hours by the time they found the body. Since the early hours of the morning, sheets of water had been swirling into the valley, soaking the corpse and the ground around it. Pools of water had gathered in the hollows of the fields below Longstone Moor, and a new stream had formed between two hawthorns, washing their roots bare of earth.

Detective Sergeant Diane Fry wiped the rain from her face and cursed under her breath as she watched the medical examiner and an assistant turn the body on to its side. Rivulets of blood-soaked water streamed off the sleeves of a green coat the victim was wearing. A crime-scene photographer crouched under the edge of the body tent to capture the moment. Big, fat drops bounced off his paper suit, ricocheting like bullets.