‘It’s increasingly common for young males to access this kind of material,’ DC Forbes told us. ‘It can become quite a compulsion for teenage boys. It becomes addictive. I’m not suggesting this was in any way connected with Billy’s disappearance but it has been noted in his file.’

Mark wanted to know what kind of hardcore porn Billy had been watching and DC Forbes was quick to reassure us that it was nothing illegal but it was quite extreme.

‘What about his mobile phone?’ I asked. ‘Have you found it?’

He shook his head. ‘GPS tracking failed to reveal anything and triangulation showed that it was last used in this house or street. We haven’t located it yet, I’m afraid.’

‘So nothing in his room has given you any clues what might have happened to him?’

‘No, Mrs Wilkinson, I’m sorry.’

I push open the door to Billy’s room and inhale deeply but all trace of him is gone. I used to tell him off for piling up his stinking trainers behind his bedroom door because you could smell them from the landing. There were other smells too: unwashed clothes, half-eaten burgers sweating in their white polystyrene boxes shoved under the bed, and the pungent chemical scent of his thick-nibbed marker pens.

I rifle through Billy’s bookcase but there’s no sign of the missing photo album amongst the neatly stacked comic books, graphic novels and the incongruous pile of Harry Potter books we used to read together before bed. When he turned eight he told me that being read bedside stories was babyish but he still insisted on Harry Potter each night. We made it all the way through the Deathly Hallows. I like to think he did that for me.

As I yank open the drawer to his bedside table the sketchbooks that are piled up on the top spill to the carpet. I pull one onto my lap and flip through it. Unlike reading, Billy’s interest in drawing has never faded but it’s been a long time since he’s drawn robots, dinosaurs and flying cars. For the last couple of years he’s done nothing but scrawl graffiti tags over every available surface.

Fliy – that was the tag he came up with first, but he changed it to DStroy when Jake teased him that he wanted to be called Fliy because he made a lot of noise and was dirty and annoying.

And here it is, page after page of thick black scrawl. DStroy. DStroy. DStroy. The letters becoming more and more illegible, turning into a spiky dark hieroglyphic as he worked on his design. He made no attempt to hide the fact that he was DStroy – that’s why it was so easy for the headmaster to identify him as the culprit behind the graffiti at school.