I hang up. I text the school again, giving them Alastair’s name and my permission. Please tell him just to walk Charlie straight home. Charlie has his own key and I’ll be there as soon as I can.

It’s okay, I tell myself, in the scratched mirror of the station toilets. My face is so thin with worry and guilt. It’s all right, I say to her, this woman that I’m trying so hard to be kind to. The school knows what to do and Alastair will be there.

Back out on the platform, I sit down on one of the hard benches, lean my head down between my knees. It will be okay. This happens to everyone at some point, I tell myself, and you’ve always done your best. Don’t be hard on yourself. Don’t be too hard.

When a train eventually arrives, racing up from Brighton, my journey home is only forty-two minutes late. As I climb aboard I do the calculations over and over in my head; this long on the train, this long on the Tube to Brixton, this long to run, as fast as I can, up the hill. I think I can get home by five past four at the latest, and earlier if I’m lucky and if everything flows on time. Charlie will only have been waiting half an hour and he’ll have someone with him all of that time. He will be okay. He will forgive me. I keep telling myself that, trying to calm down, even while all the way back on the train, I am counting every second.

The half-hour it takes to get back to Victoria feels like an eternity, each minute stretching my nerves to the limit. I pull out my phone to text the school again, but when I press the button to light the screen instead it gives a painful bleep and switches itself off. No battery. The phone is old and doesn’t hold its charge and like a fool I’ve not brought my charger with me. I can’t update Alastair, I can’t update the school. All I can do is sit rigidly in my seat, willing the wheels of the train to turn faster.

Still miles away from Victoria, I pull my bag up from where it lies tangled with my feet, leave my seat and go and stand in the vestibule, so I can be the first one off. Hopelessly, I check my phone again, but now it won’t even switch on. I’m just glad I managed to send the texts I did before it died.

Victoria Station is heaving, but I push through the crowds. Each step takes me closer and I luck out in the Tube, stepping onto the platform just as the train for Brixton is pulling in. I head down to the end and find space in the last carriage.