My mother, being German, knew how to do a good Christmas. It was her speciality. The house was festooned from the beginning of December with homemade decorations made of candied oranges and red gingham and painted pine cones and filled with the aroma of gingerbread, stollen and mulled wine. No tacky tinsel or paper garlands for her, no tin of Quality Street or Cadbury’s selection boxes.

Even my father enjoyed Christmas. He had a Father Christmas outfit which he used to don every Christmas Eve when we were little, and I still can’t explain how I could both know it was him, but also have no idea it was him, at the same time. Looking back on it now, I can see that it was the same sort of terrible self-deception that played a part in the way everyone felt about David Thomsen. People could look and see just a man, but in the same glance, the answer to all their problems.

My father didn’t wear the outfit that Christmas Eve. He said we were all too old for it, and he was probably right. But he also said he didn’t feel too well. My mother laid on the usual Christmas Eve celebration anyway. We sat around a (smaller than usual) Nordic pine and unwrapped our (fewer than usual) gifts while Christmas carols played on the radio and a fire crackled in the grate. After about half an hour, just before dinner, my father said he needed to go and lie down, he had a terrible headache.

Thirty seconds later he was on the floor of the drawing room, having a stroke.

We didn’t know it was a stroke at the time. We thought he was having some kind of fit. Or a heart attack. Dr Broughton, my father’s private physician, came to look him over, still in his Christmas Eve outfit of red woollen V-neck and holly-print bow tie. I remember his face when my father said that he no longer had private health cover, how quickly he left the house, how he dropped his unctuous demeanour like a brick. He sent him straight to hospital in an NHS ambulance and left without saying goodbye.

My father came home on Boxing Day.

They said he was fine, that he’d have some cognitive challenges for a while, some motor problems, but that his brain would fix itself, that he would be back to normal within weeks. Maybe sooner.

But, as with his first stroke, he never recovered properly. There was an even greater vacancy there. He used the wrong words. Or couldn’t find any words at all. He spent whole days sitting in the armchair in his bedroom eating biscuits, very slowly. Sometimes he’d laugh at inappropriate junctures. Other times he wouldn’t get the joke.

He moved slowly. He avoided stairs. He stopped leaving the house entirely.

And the weaker my father became, the closer to the mark David Thomsen stepped.

By the time I turned fourteen in May 1991, we had rules. Not just normal family rules like no feet on the furniture, or do your homework before you watch TV. Not the sort of rules we’d had for all our lives.

No, now we had crazy, despotic rules, written out in black marker on a large poster that was taped to the kitchen wall. I can still remember them to this day:

No haircuts WITHOUT PERMISSION of David and/or Birdie

No television

No visitors WITHOUT PERMISSION of David and/or Birdie

No vanity

No greed

Nobody to leave the building without the EXPRESS PERMISSION of David and/or Birdie

No meat

No animal products

No leather/suede/wool/feathers

No plastic containers

No more than four pieces of rubbish per day per person, including food waste

No unnatural coloured clothes

No pharma

No chemicals

One wash or shower PER PERSON PER DAY

One shampoo per week

ALL RESIDENTS must spend a minimum of two hours a day with David in the exercise room

ALL CHILDREN must spend a minimum of two hours a day with Birdie in the music room

All food must be homecooked from organic ingredients

No electric or gas heating

No shouting

No swearing

No running

This list of rules had started quite short and was added to at intervals as David’s control of our household got stronger and stronger.

Sally, at this stage, still used to come to the house once or twice a week, to take the children out for tea. She was sleeping on a friend’s sofa in Brixton and desperately trying to find some kind of accommodation big enough for them all to live in. Phin would be extra sullen after spending time with his mother. He would lock himself in his room and miss the next couple of meals. It was because of Phin, in fact, that a lot of the rules were put in place. David found his moods untenable. He could not bear the wasted food, or the door which he was unable to open at will. He could not bear anyone doing anything that did not directly correspond with his own view of the world. He could not bear teenagers.

Two new rules were added:

No locked doors

ALL MEALS to be attended by ALL members of the household

One morning, shortly after the fifth time Phin had come back from spending the afternoon with his mother and broken the rule about ‘No locked doors’, I went upstairs to find David removing the locks from inside Phin’s room, his jaw clenched, his knuckles tight around the handle of the screwdriver.

Phin sat on his bed, watching with his arms folded hard across his chest.

When at dinnertime Phin was still sitting on his bed with his arms folded, silent and deathly, David dragged him down by his – still folded – arms and threw him into a chair.

He forcibly pushed the chair into place and served Phin a large bowl of curried marrow and rice. Phin’s arms remained crossed. David got to his feet, piled some curry on to a spoon and forced it to Phin’s lips. Phin locked his lips together. I could hear the spoon hitting his teeth. The atmosphere was shocking. Phin, at this stage, was fifteen and a half, but looked much older. He was tall and he was strong. The situation looked as though it could turn violent very easily. But Phin stood his ground, his eyes boring a hole into the wall opposite, his whole face rigid with anger and determination.

Eventually David gave up trying to feed the spoon into his son’s mouth and hurled it across the room, the curry forming an ugly yellow crescent across the wall, the spoon making an angry metallic scream as it hit the floor.

‘Get to your room!’ David shouted. ‘Now!’ A vein throbbed on his temple. His neck was tensed and puce. I had never before seen a human being as engorged with rage as David at that moment.

‘With pleasure,’ hissed Phin.

David’s hand appeared; then, almost in slow motion, as Phin passed him it connected with the back of his head. Phin turned; his eyes met his father’s eyes, I saw true hatred pass between them.

Phin carried on walking. We heard his footsteps, sure and steady up the staircase. Someone cleared their throat. I saw Birdie and David exchange a look. Birdie’s look, pinched and disapproving, said, You’re losing control. Do something. David’s look, dark and furious, said, I intend to.

The moment the meal was over I went to Phin’s room.

He sat on his bed with his knees drawn tight to his chin. He glanced up at me. ‘What?’

‘Are you OK?’

‘What do you think?’

I edged a little closer into the room. I waited for him to ask me to leave but he didn’t.

‘Did it hurt?’ I asked. ‘When he hit you?’

My parents, strange as they both were, had never hit me. I couldn’t even imagine such a thing.

‘Not really.’

I edged closer again.

Then, suddenly, Phin looked up at me and it was there again. He was seeing me. Properly.

‘I can’t stay here,’ he said, shaking his head. ‘I’ve got to get out.’

My heart skipped a beat. Phin was the only thing that kept any sense of possibility alive.

‘Where will you go?’

‘I don’t know. To Mum’s.’

‘But—’

I was about to say that his mum was sleeping on a sofa in Brixton. But he interjected. ‘I don’t know, all right? I just have to get out of this place. I can’t be here any more.’

‘When?’

‘Now.’

He looked at me through his ridiculous eyelashes. I tried to read his expression. I felt I saw a challenge there.

‘Do you … Should I … come with you?’

‘No! Fucking hell. No.’

I shrunk back into myself. No. Of course not.

‘What shall I say? When the adults ask?’

‘Nothing,’ he hissed. ‘Just nothing. Don’t say anything.’

I nodded, my eyes wide. I watched him throw things into a drawstring bag: pants and socks, a T-shirt, a book, a toothbrush. He turned and saw me looking at him.