My father never spent Christmas Eve at home with us. He spent it with Anne and the twins, his first family, the family he had before he met my mother. We almost never talked about them at home; my mother felt threatened by their very existence, which was strange considering she had been the cause of their unhappiness.

"You know very well they can't, " replied my father, using the tone of quiet tension that pervaded their marriage. "They'd only argue."

"But it's so expensive buying everything twice, " she sighed, walking away.

I knew that my father regretted the swap he'd made. When one of the twins called him, he stayed on the phone for longer than he ever did with anyone else and came back into the living room with tears in his eyes. On the rare occasions that he spoke of his first wife, he portrayed her as a sainted creature, cruelly betrayed by his own weakness.

I made a point of buying presents for my halfbrothers and placing them in the bags before he left the house. He never said anything about this to me but, when my mother was out of sight, he handed me a single parcel from the twins and put a finger to his lips to let me know that this was a secret between the two of us. I opened the present privately, afraid to let anyone know of our clandestine exchange. The twins always spent a lot of money on me, although I knew that it wasn't really the twins at all, it was Anne.

When I was nine I asked my father whether we could all have Christmas dinner together and he laughed at first before shaking his head and ruffling my hair. "Only if you want a massacre over the turkey and stuffing, " he said.

One year my mother discovered me leaving the twins' presents in the bag and she stared at me for a very long time before closing her eyes and breathing heavily, as if she was slowly counting to 10 in her head. "This is a betrayal, " she said. "You might as well have stuck a knife in my chest." I was lucky. If she had reached down a little further she would have found a present for Anne too. It was the same present I'd bought her.

I'd got them in a 2-for-1 at Boots.

He left the house with his black bags of presents every Christmas Eve after lunch and didn't come home until the early hours of the morning when I was already fast asleep. For years, I thought there was a good chance that he was really Santa Claus.