On Friday afternoon, when I collect Charlie from school, I finally make good on my promise and take him to the play park before we go home. As we head down into Brockwell Park, I take his backpack and swing it onto my shoulder, pull a pair of gloves out for him from my handbag and make sure his coat is fastened right up to his chin.

Charlie holds me by the hand as we walk, tugging on my arm with each step. It is crisp, the wind chill, and I pull the collar of my coat up, Charlie’s backpack slipping a bit as I do.

When we get to the park, Charlie wants to go on the swings, his favourite, and he doesn’t need me to push him, he can do that easily himself. I stand beside the swing frame, leaning on the cold, scratchy pole.

‘Watch me!’ Charlie says, and he clambers onto the seat. ‘Watch me!’

I am watching. I am always watching him. Sometimes I am watching him so closely that I’m not even really seeing him. I’m listening so carefully to how he speaks, and scanning every element of his coordination, and trying to judge whether every single one of his emotions seems normal for his age.

When he’s set in the swing he turns and gives me a big smile and I give a big smile back.

‘Look!’ he shouts. ‘Look how high I can go!’ He leans back, kicks his legs, hangs his head back. On the swing, he looks perfect; it seems impossible to me right then that anything could be wrong. I blink my eyes, trying to clear the fog in my head and clap my gloved hands, a muffled thump in the cold.

‘Great job!’ I shout as he kicks himself higher still. Look at him, I tell myself. He’s gorgeous. He’s fine.

Half an hour later, after he’s been on the roundabout and the slide, and played tag with a group of other children too, it is growing cold. I can see that – despite the coat and the gloves and the running about – Charlie’s lips have a bluish tinge.

I hold out his backpack for him to put on.

‘Come on, Champ,’ I say. ‘Time to go home.’

Charlie is babbling away to me as we turn into our own street and I’m looking down, half-enchanted by him so that at first I don’t see her. Then I glance up. The stocky woman outside my flat turns towards me and I pull up short, clenching Charlie’s hand in mine.

I recognize her. Now I remember.

‘Jennifer?’ she says. I check my watch. Four forty. I forgot. How on earth could I forget?

She looks chilly, her dark skin paling in the cold. I wonder how long she – Margaret, I remember her name now – has been standing out there, on the pavement.