More horses followed into the killing room. A large grey, a small black pony. The slaughterman was good. He shot each animal one-handed, smack in the middle of the forehead, and each one dropped instantly, dead but for the spasmodic twitching of their legs. The man even took time to chat to the horse handler as he manoeuvred a horse into the killing position: ‘Did you watch the racing yesterday?’

The strangest thing was the sound of Radio Two playing somewhere in the background. At one point, a door out of camera range must have been opened, releasing a louder blast of Abba. ‘Money, Money, Money’ or ‘Gimme, Gimme, Gimme’. He half expected the slaughterman to burst into song himself, maybe to do a little dance in his white apron and cap. His work was almost choreographed, so it wouldn’t have been totally unfitting.

The link to the film had been sent to him in an email. The sender’s address was one of the free web-based email accounts, which could be set up without providing a postal address or a phone number, or even a real name. If you wanted to make sure you stayed anonymous, you could create an account specifically for sending one email. Then you sent it on a public access terminal in a library or internet café, and closed your account. He could attempt to get the sender traced, but it was probably futile.

He couldn’t figure out why some of the footage had sound, but other sections didn’t – even though they were obviously shot from the same place. At one point, a door partially blocked the view of the camera, resulting in a glimpse only of a horse’s back legs thrashing on the floor, until they gradually came to a halt.

Cooper had grown up on a farm. He knew that animals had to be killed, for all kinds of reasons. As far as he was concerned, there should be no problem with that, so long as it was done properly. Quickly, efficiently and humanely. Those were the key words. He knew that most slaughtermen took pride in doing their job well, so that an animal didn’t suffer unnecessarily. But it was a thankless role, one that the public at large would rather pretend didn’t exist. That essential stage between the cute animal skipping around a field and the joint of lamb on a supermarket shelf was best left unexplored. What you didn’t know, didn’t hurt.

‘C.J. Hawley and Sons,’ said Hitchens. ‘Do we know anything else about them?’

‘Only that theirs was one of the numbers on Patrick Rawson’s calls list,’ said Cooper.

The rest of the film showed footage of a sick or injured horse lying in a yard outside the abattoir. Its head rolled, and it tried to sit up a couple of times, but gave up. Meanwhile, a man could be seen walking backwards and forwards past it, talking on a mobile phone. Eventually, he came back with a gun and shot the horse in the head. He was in ordinary casual clothes, a blue check shirt and a pair of worn denim jeans, and it was difficult to say whether this was the same man who’d been filmed in the killing room.