Ten minutes later, the Tube train jolts to a stop, the blips sound and the doors slide open. We’re here, my home stop, Brixton.

I run up the left-hand side of the escalators, take the steps to the street two at a time. My coat is thick and cumbersome for the cold weather and my handbag with its too-short straps keeps slipping off my shoulder. In the shoes I’m wearing, I risk wrenching an ankle, but I make myself run all the way up the hill, only slowing near the top, my side wrenched by an agonizing stitch. But I am so nearly there, and it’s barely five to four. I have made such good time, Charlie will still be at the school, with his teacher. I can relax; it’s going to be all right.

I take the back street off Brixton Hill, the one that leads straight to the school. When I make it to the playground, one minute to four, there is no sign of Charlie, but then of course, they wouldn’t be waiting outside. It’s too cold for that. I push at the front doors of the school to head inside. I know where Charlie’s classroom is, it’s one of the first off the main corridor. I know exactly where he will be.

But the outer doors jolt against my hand. Locked. I can’t get in.

I try again, shaking the handle, but it really is locked, the school is closed. Now I can tell – there are no lights on inside. And there is no sign anywhere of Charlie.

Home then. Alastair must have taken him home. He must have arrived early, before four, and why would a teacher wait past then? I turn to head to our flat and, as I do, in the staff car park I see a little blue car, just leaving. It stops when the driver sees me and the woman inside – I recognize her now as Charlie’s teacher – rolls down the window.

‘I’m so sorry,’ I say. ‘I couldn’t … my train … and then my phone …’ I stop and take a breath. ‘Has Charlie gone home?’

Ms Simmons smiles at me. ‘You didn’t get my text? Your friend came … Alastair. Charlie was fine and went with him.’

‘All right, thank you. I’ll head straight there. Thank you, I promise, it won’t happen again.’

I stumble-run up the road, my legs shaky beneath me. Ms Simmons shouts after me. When I look back, she’s pointing – they went that way! – but that doesn’t make any sense. Every second feels too long, but I am nearly there, I am nearly with him. Here is my flat, and I already have my key in my hand as I run up the steps. I shove open the door, half falling inside, calling out for them. ‘I’m here!’