For days after that, all I could think about was Ella and that poor baby, Catherine, and what she did to her. It was unbearable. It made me shiver each time I thought about how much you’d known, so long before I did, and the innocent way you’d described your new friend with ginger hair.

Richard suggested dates for a holiday in July. He wanted to book, he said, to take you away to Spain. The three of them. Sea and sand and buckets and spades. You’d love it.

When I tried to stall, to say I wasn’t sure, I needed to think about it, his tone turned cold. There’s nothing to think about, Jen. You took her to Venice with your boyfriend. What’s the difference?

How could I explain? How could I tell him that if Ella was capable of doing such a monstrous thing to her own child, she might hurt you too? I couldn’t sleep for worrying.

Matt was right. I knew it. If I wanted to stop her, I needed evidence. And finally, my chance came.

The following weekend, Richard invited you for another sleepover. This time, instead of letting him collect you, I insisted on taking you to their house myself. It was your idea, I lied. You were desperate to show me the bunk beds. You’d talked of nothing else.

Richard sounded hesitant. I knew why. She didn’t like me visiting their house. But I insisted and he didn’t have much choice.

I played one of your CDs in the car and while you sang along in the back, your voice high and off-key, I tried to relax, to steady my breathing and think through what I needed to do. The car stuttered and jolted through heavy traffic.

When Richard first left, he moved into Ella’s flat. I picked up bits and pieces from little things he said, from the background noises when we talked on the phone. It was a tiny flat in the heart of the city, throbbing at night with live bands and rowdy drunks outside.

You were only just walking then but even at that age, you seemed fascinated by Ella, with her tight clothes and glittery shoes. It led to petty battles. Sour notes from me when you returned one day with your thumbnails painted red and, another day, with the temporary tattoo of a winged horse on your thigh.

It was a second-floor flat with steep stairs. A nail salon on the first floor. Two men above who worked nights in a bar. It made me laugh at the time. Bitterly, of course. It was hard to imagine a place less like Richard – my dear, stay-at-home, prematurely middle-aged husband. But then he wasn’t any more, was he?

Eventually, the two of them took the next step and moved into a small Victorian terraced house in the suburbs, closer to us. A big lifestyle change for Ella. I assume Richard persuaded her. They needed more space and a second bedroom for you. He tried, I give him that. I’d seen it from the outside once or twice when I dropped you off but she’d never invited me in.

Now I turned off the main road, with its parades of grocery shops, newsagents and dry cleaners, the streets became residential and quiet. A smart young woman, earphones in her ears, pushed a buggy. A traffic warden passed, strolling, watchful, punching registration numbers into a hand-held device. Further down the road, there was a clatter of metal as builders unloaded poles from the back of a truck.

The blood banged in my ears as I approached number forty-two and found somewhere to park. When we got out, I paused at the hedge for a moment, looking it over. The gate was latched. The lid of the bin was pushed up by a bulging bag inside. Upstairs, the bedroom curtains were drawn back but the windows were dark.

Richard answered the door. You ran in past him, your bear under your arm, shouting hello to the quietness. A moment later, you shed your coat, leaving it on the floor, and went clomping up the stairs as fast as you could.

Richard and I looked at each other. His face was strained.

I managed to smile. ‘What about these bunk beds? Can I have a look?’

‘I told you.’ He shrugged, stood aside to let me squeeze past. ‘They’re perfectly safe.’

I crossed the bare-wood floor of the hall, wondering if I was supposed to take off my shoes. The door to the left stood open and I paused for a glimpse inside at a large, open-plan sitting room. The walls and carpet were cream, the furniture all glass and metal. It was sparse and modern, a stark contrast with our home, which was a clutter of tired wooden furniture inherited from family over the years or picked up second-hand. Our interiors weren’t designed, they just evolved.

I looked at the pristine surfaces and imagined you crayoning on the walls or running over the carpets in muddy shoes and thought what pleasure it would give me if you did.

‘No Ella?’

‘She’s popped out.’

I bet she has. Waiting until the coast’s clear. He didn’t offer me a drink, just pointed the way up the stairs after you, as if he wanted this over and done with as soon as possible.

There was a small first landing with a bathroom and a closed door. I carried on to the next level, following the sound of your voice as you called to me.

There was no doubt which room was yours. You’d already kicked off your shoes and were crouched on a fluffy pink rug in the shape of a bear, playing with a patchwork elephant. The walls were cream, decorated with framed pictures of fairies and princesses. Behind you, against the wall, were the new bunk beds with their carpeted stairs. The duvet hung over the edge of the top bunk. The cover showed rainbows and unicorns.

The lower bunk was piled with stuffed animals. A spotted horse. A family of rabbits in dresses, with patchwork ears. A teddy with a pink bow. Everything looked brand new. I set your overnight bag down in the corner and went to sit beside you.

‘Look, Mummy!’ You scrambled to your feet and crawled across the lower bunk to disappear into the hidey-hole under the stairs. Your voice came out: ‘Where am I?’

‘I don’t know,’ I said, playing along. ‘Where’s Gracie gone, Daddy?’

Richard stood awkwardly in the doorway, his shoulders hunched. He didn’t answer. He looked embarrassed. The expense, perhaps. Your room here was so much more extravagant than I could give you at home. The liberal splashes of pink were tacky but you were three; of course you loved it. I knew exactly who was behind it all. It was another weapon in her battle to seduce you, to win you over.

I smiled at him. ‘Do you mind if I play with her for a bit? I won’t stay long.’ I lowered my voice to a whisper to add: ‘She’s been so keen to show me.’

‘If you want.’ He didn’t look pleased but he gave in and turned away. I listened to his heavy footsteps on the stairs, heading back to the ground floor.

As soon as he’d gone, I whispered to you: ‘Gracie, I need you to be very good. Can you do that? Mummy’s got to go and do something.’

You poked your head out, uncertain. I kissed your nose.

‘You play quietly here with elephant and horse and teddy,’ I said. ‘Until I come back. OK?’

I left you chattering to your toys while I crept across this second landing to the main bedroom at the front of the house. I faltered on the threshold and steadied my breath.

My eyes went at once to the bed. A king-size, unmade, the duvet thrown back, as if someone had just climbed out. The sheets crumpled. I crossed to it, found my feet caught up in a furry bedside rug. Hers. Not Richard’s style. I rubbed the edge of the sheet between my fingers. Cool and crisp. Good quality cotton. Our sheets at home were soft with over-washing.

There were fitted wardrobes down one wall. I opened them quietly, door by door, and looked inside, checking the floor space for boxes, for files.

His suits hadn’t changed much but he wore pure silk ties now in brighter colours and bolder designs. Pale pink and blue shirts hung alongside the white. That was her influence.

Her wardrobe had slinky cocktail dresses. At least six. Two long evening dresses, shrouded in dry cleaner’s bags. Waterfall cardigans in cashmere. An angora wrap. Crisp shirts and blouses in all shades. Beneath, neatly arranged on a chrome rack, about a dozen pairs of evening shoes. Long spiked heels and spaghetti straps.

I closed the wardrobe doors and got on my hands and knees to check under the bed. A noise, just outside the house. The click of the front gate. I scrambled to the window to look, standing back against the curtain to peer down. The postman’s red trolley stood on the pavement.

Downstairs, the letter box clanged, then a smack as post hit the mat. A moment later, the postman went back down the path and wheeled his trolley forward a few paces to next-door’s gate.

My stomach was tight, my skin hot. I stood still, listening. Richard’s tread on the wooden floor as he crossed the hall to pick up the letters. My heart thumped. The footsteps faded again, back to another part of the ground floor. I needed to be quick.

The bedroom was unnaturally bare, so unlike my own. I had a whole suitcase full of your baby things. Then there were my old diaries, boxes of school reports, photographs. I even had a box under the bed with souvenirs from our wedding, right down to the ribbon from the cake.

I crossed to the chest of drawers and rummaged through the drawers. Tights and stockings in packets. Richard’s cotton boxer shorts and boxes of cuff links.

Then a drawer of her silky underwear. I tried not to imagine her wearing it. Not to imagine Richard taking it off. I reached a bottom drawer when my fingers felt something different, something cool and hard. I crouched down and pulled out the drawer as far as it would go. It was her overflow space, filled with suspender belts, jewellery boxes, handkerchiefs and purses. I lifted them out in handfuls. Right at the back was a slim, clear plastic folder.

I first at thought that the photographs, in plain cardboard mounts, like old-fashioned school portraits, were remnants from another generation. The pictures were black and white but seemed muted. There was also a stillness about them, a timelessness. I sat there on the carpet, gazing at them.

They were Ella’s features but transformed by such tenderness that I barely recognised her as the woman I knew. Her shoulders were covered by the lacy sleeves of a nightdress or bed jacket. Her hair was loose round her neck.

In one, she gazed with wonder at the baby in her arms. The scrunched face of a newborn with closed eyes. Another picture showed the baby’s tiny hand curled round her manicured finger. The final image was of the baby, clothed in a sleep-suit and a delicate lace-edged bonnet, fast asleep in a Moses basket. A printed slip inside the frame read: Celebrating the birth of Catherine Louise. The date was eight years ago. The final item in the folder was wrapped in tissue paper. A tiny lock of ginger hair.

I tore the paper label off the back, with the name, address and telephone number of the photographer, and shoved it inside my pocket. The intensity of feeling in Ella’s eyes made me physically sick. It was impossible to reconcile that love with the knowledge of what she did to her, so soon after these pictures were taken. I shook my head. The photographer who framed those artistic portraits would be just as shocked.

‘Richard?’

Her voice, calling, down in the hall. The slam of the front door behind her. I stuffed the pictures, the hastily re-bundled hair and tissue paper, back into the plastic folder, threw the handkerchiefs and jewellery boxes on top and closed the drawer. It shut with a bang that sounded deafening in the quiet.

A creak from the stairs. I bolted across the landing, back to your room.

‘Where were you, Mummy?’ You looked cross. You didn’t like being on your own.

I picked up the first stuffed toy I saw and started to talk to it, pretending we were in the middle of a game.

‘Yes, Mr Elephant,’ I said, ‘of course you can have tea too. Do you take milk?’

My breath was short and the words came out in gasps. You narrowed your eyes.

‘What, Mummy? What are you doing?’ Then you looked past me, and smiled. ‘Auntie Ella!’

She stood there, in the doorway. Watching me. Her eyes were cold.

You jumped up and ran to hug her, and her eyes softened as she folded her arms round you and hugged you back.

I felt my cheeks flush. The stolen label from the back of the pictures burned inside my pocket. I was afraid to move quickly in case it fell out. I thought of the plastic folder, shoved back so quickly, and the belongings heaped on top, left in disarray.

The silence stretched. She waited, her forehead tightened as she looked down at me. Finally, she said: ‘I think it’s time you left. Don’t you?’