Today, though, there had been arousal and erection, a wonderful erection. It was still possible. He still functioned, and that was, indisputably, good news. Each time he remembered the erection he felt jubilant – an elation qualified by an ill-defined sense of wrongdoing.

Cycling home that night, Callum tried to parse his fantasy more definitively. What was the locus of his excitement? Perhaps it was the idea of looking at Tamsin’s body in an inappropriate space that turned him on. Some people fantasised about masturbating in public; this must be a variant of that.

But his behaviour had been undeniably reckless. From now on, he told himself, there would be no more mid-lesson viewing sessions.

* * *

At the end of January, Callum and Tamsin were still together. Every Monday Tamsin told herself that she would do it at the weekend; and then it was Friday, and then it was Saturday, and then it was Sunday and she hadn’t done it. There was always some reason for her deferral, although these reasons were never good enough to save Tamsin from feeling like a coward on Monday mornings. Now it was mid-February, and today, Saturday, was Valentine’s day. Obviously you couldn’t break up with someone on Valentine’s day.

‘It’s Valentine’s day today,’ she said brightly as she entered Antonia Platt’s bed-sitting room. ‘Look, I brought us some cake.’ Tamsin showed Antonia the soft mushroom-clouds of two raspberry and white chocolate muffins, topped with a sprinkling of tiny pink sugar hearts.

The television was showing a Wallace and Gromit movie. Tamsin turned the volume down but left the movie playing in the background, to fill the long, difficult silences that increasingly punctuated her visits to Antonia.

‘Oh, you’re wearing gloves?’ Antonia asked. ‘I haven’t started that yet. I’m going to wait until the weather gets a little colder.’

‘With any luck it should be getting warmer,’ Tamsin said mildly. She had learned never to sound as if she were contradicting or correcting the old lady. Antonia’s condition had worsened rapidly over the last couple of months, and as meaning receded, she was becoming extremely sensitive to tone of voice. Before Christmas, her blank moments – like the confusion over the flutes – had been intermittent. She had a tendency to repeat herself, but back then it was still possible for Tamsin to cut in after a sentence or two and say, ‘Ah, yes, I remember you telling me about that.’ About a month ago, Tamsin had stopped bothering. It was easier and nicer for both of them if she simply let Antonia tell the story again, and again. Tamsin also found herself answering incessantly repeated questions as if she had never heard them before. Last week, for example, she had given Antonia her address in Notting Hill four times, each time agreeing with credible surprise that it was indeed a great coincidence that Antonia had been born just a few streets away from Ashcombe Mews.