In the ten-minute clip, a boy and girl – or should he think of them as young man and woman? – shagged each other in what was actually a pretty vanilla way. A strip, a quick blow job, followed by missionary position sex and a bit of doggy style on a double bed. As porn went, it was at the softer end of the scale. What made it remarkable though was the fact that, until they stripped, the fornicating couple were wearing school uniforms, and on their heads they wore masks to conceal their identities. Because he couldn’t see their faces, Winkler couldn’t tell exactly how old they were, though he would guess from their bodies, and from their voices as they spoke a few lines of clichéd dialogue, they were no more than fifteen or sixteen. A quick Google image search told him the uniforms were, as he suspected, from St John’s.

By the time he’d finished watching the video, Hattie Styles was no longer online. He printed out all her messages – and, more crucially, his own replies masquerading as Helen – then deleted them from Helen’s Facebook inbox. Then he sat back and thought about what all this might mean and how it could be connected to the investigation. The girl in the clip was definitely white, so it wasn’t Alice. But ‘Hattie’ had said Alice was responsible for the video. So . . . what, she’d filmed it? Were she and her boyfriend amateur porn directors? Good grief, teenagers today. When he was a kid, the worst things he ever did were shoplifting seven-inch singles from Woolworths and getting into the odd scrap with lads from the rival school. The closest he came to porn was passing round a contraband copy of Penthouse with his mates and marvelling at the bushes. Now, though, he lived in a world where women didn’t have pubes and every teenager in the western world had instant access to every variety of hardcore porn ever created. He sighed. This generation was so fucking lucky.

Now, he walked across the grounds of the school towards the reception. It was that time in the summer term when most pupils had finished their exams and there was a giddy quality in the air. Winkler felt like he had a hand grenade in his pocket that would destroy all that carefree good feeling. Pull the pin and boom! He had a Tigger-ish bounce in his step as he buzzed for the receptionist to let him in.

Five minutes later he sat in the head teacher’s stuffy office drinking a lukewarm glass of tap water. The head teacher, Hazel Fletcher, was a smart white woman with a golden bob who reminded him a little of Helen Mirren. A silver vixen. He felt only slightly guilty about ruining her day.

‘I’ve found out something involving a couple of your pupils that might shock you,’ he said.