There was a time when I would have found a group of young lads like this threatening. I’d have crossed the street rather than risk attracting their attention but I’ve stopped being scared. I don’t care if they think I’m old and embarrassing and uncool.

I rest my hand on the column. It feels cold and damp under my palm. ‘Do any of you know Billy Wilkinson?’

‘Why?’

‘He’s my son. He’s been missing for six months.’

‘I know him,’ pipes up the smallest boy in the group. It’s the first thing he’s said since I appeared in the clearing but I’ve seen him watching me, tracking me with his half-closed eyes.

‘Shut up, Gray. We don’t know who this woman is. She looks like police.’

‘Undercover mum,’ says Spliff Boy.

The lad with the beer can swipes him round the head. ‘Don’t be a dick.’

‘This is Billy.’ I unfold the flier I carry everywhere with me, and take a step towards them. It’s very nearly dark now, the last of the sunlight is fading away and they peer through the firelight at the image of Billy’s face. ‘He’s fifteen. He does … he’s really into graffiti. Have you seen him? Recently, I mean?’

My question is met with shrugs and glazed looks.

I step around the group and crouch down next to Gray. The air surrounding him is thick with the scent of woodsmoke, weed and beer.

‘You said you knew him. How?’

He inches away, pressing up against the boy sitting next to him.

‘I know of him,’ he says as he’s shoved away. ‘I heard of him.’

‘How?’

‘From the news, like.’

‘Are you sure?’ I look him straight in the eye but he’s unable, or unwilling, to meet my gaze and he fiddles with the laces of his trainers. ‘Please, it’s important. I know he’s been here before. I know he wanted to tag the bridge. Have you seen or heard anything unusual?’

‘Naz’s face is unusual,’ says Spliff Boy and they all laugh. Everyone apart from Gray who is twisting a lace round and round the index finger of his left hand. He’s hiding something from me. If the others weren’t here he’d tell me the truth. I feel sure of it.

I dip my head down to his and lower my voice. ‘Could I just talk to you? Alone? Just for a minute?’ I touch his shoulder and he jumps away from me, as though electrocuted, narrowly missing the fire as he scrambles to his feet.

‘Whitey alert!’ shouts Naz as Gray runs towards the river, then drops to his knees and pukes all over his hands. My heart sinks. He wasn’t hiding anything from me – he was trying not to be sick.