I don’t think twice. I indicate left and pull into the bus lane. A horn sounds behind me and the driver of the 3A bus shakes his head at me in my rear-view mirror. Startled by the sound, the man on the bike glances back. I wave frantically but he either doesn’t see me or he doesn’t want to stop because his head drops and he begins to pedal even faster. He turns left onto Zetland Road just as the lights change and I’m forced to stop.

I drum my fingers on the steering wheel as he zips across the road and jumps off the bike outside a kitchen-and-bathroom shop and then hammers on the panelled wooden door of the building next to it, on the corner of the street. There are curtains at the window and a large piece of white card or wood – at least twelve feet by six feet – propped up inside, obscuring the view. As the traffic light turns green the door opens and the man disappears inside, taking the bike with him. It has to be the squat Jake told me about.

There’s a space outside a tile shop on the opposite side of the road so I park quickly, half mounting the pavement in my desperation to get out of the car.

I have to wait for one, two, three cars to go past before there’s a gap in the traffic and I can sprint across the road.

‘Hello!’ I knock on the door and then wait.

A young mother walks past, pushing a red-faced, squalling baby in a pram. Her eyes are fixed on a spot in the distance, as though she’s willing herself to … just … get … home. She doesn’t so much as glance at me.

I knock again and walk around the corner and tap on the window.

Nothing happens. No one comes to the door and the curtains don’t twitch.

‘Hello?’ I lift the letterbox and peer inside but it’s lined with nylon bristles and I can’t see a thing. ‘Hello! I know you’re in there. I just saw you go in with the bike.’

‘They’re all drug addicts, you know.’ An elderly man, with a walking stick in one hand and a blue plastic bag in the other, pauses beside me. ‘If they’ve stolen something of yours you need to call the police.’

I instinctively touch my handbag, slung across my body. I should call the police. Or at least Mark. But adrenalin’s coursing through me and I can’t stop myself from shouting through the letterbox again as the man continues his amble up the road.

‘My name’s Claire Wilkinson. My son Billy is missing. I think you might know him.’