But that first time, it was almost pitch black already, and we scampered under cover of darkness until we reached the clutch of the stunted bushes and trees that marked the edge of the marsh, and could let out our suppressed giggles, and turn on our torches.

Kate led the way, the rest of us following her through a dark maze of channels and ditches filled with black brackish water that glinted in the torchlight.

We climbed over fences and stiles, jumped ditches, paying careful heed to Kate’s muttered instructions over her shoulder, ‘For God’s sake, keep to the ridge here – the ground to the left is pure bog … Use the stile here, if you open that gate, it’s impossible to shut again and the sheep will escape … You can use this tussock of grass to jump the ditch – see where I’m standing now? It’s the firmest part of the bank.’

She had run wild on the marsh since she was a little girl, and although she couldn’t tell you the name of a single flower, or identify half the birds we disturbed on our walk, she knew every tuft of grass, every treacherous bit of bog, every stream and ditch and hillock, and even in the dark she led us unerringly through the labyrinth of sheep paths, boggy sloughs and stagnant drainage ditches, until at last we climbed a fence, and there it was – the Reach, the waters glinting in the moonlight, and far up the sandy bank in the distance, the Mill, a light burning in the window.

‘Is your dad home?’ Thea asked. Kate shook her head.

‘No, he’s out, something in the village, I think. It must be Luc.’

Luc? This was the first I’d heard of a Luc. Was he an uncle? A brother? I was almost sure that Kate had told me she was an only child.

Before I had time to do more than exchange a puzzled glance with Fatima, Kate had started off again, striding up the lane this time without looking back to check on the rest of us, now we were on firm, sandy ground, and I ran to catch up.

At the door of the Mill she paused for a moment, waiting for Fatima, who was bringing up the rear, panting slightly, and then she opened the door.

‘Welcome home, everyone.’

And I stepped inside the Mill for the first time.

It has hardly changed, that’s what’s remarkable, as I think back to that first time I saw the place – the pictures on the wall were a little different, the whole place slightly less drunken, less tumbledown, but the twisting wooden staircase, the lopsided windows casting their golden light out across the Reach, all that was the same. The October night was cold, and a fire was burning in the wood stove, and the first thing that struck me when Kate opened the door, was a blast of warmth, and firelight, woodsmoke mingling with the smell of turps and oil paint and seawater.

Someone was there, seated in a wooden rocking chair in front of the fire, reading a book, and he looked up in surprise as we entered.

It was a boy, about our age – or, to be exact, five months younger than me, as I found out later. He was actually only a year older than my younger brother – but he was a world away from little pink-and-white Will in every other respect, his lanky limbs tanned nut brown, his dark hair jaggedly hacked, as if he’d cut it himself, and he had the slight stoop of someone tall enough to have to worry about low doorways.

‘Kate, what are you doing here?’ His voice was deep and slightly hoarse, and there was a touch of something that I couldn’t place, an accent not quite the same as Kate’s. ‘Dad’s out.’

‘Hi, Luc,’ Kate said. She stood on tiptoes and kissed him on the cheek, a rough, sisterly kiss. ‘Sorry I didn’t warn you. I had to get out of that place, and, well, I couldn’t leave the others to rot at school. You know Thea, of course. And this is Fatima Qureshy.’

‘Hi,’ Fatima said shyly. She stuck out her hand, and Luc shook it, a little awkwardly.

‘And this is Isa Wilde.’

‘Hi,’ I said. He turned and smiled at me, and I saw that his eyes were almost golden, like a cat’s.

‘Guys, this is Luc Rochefort, my …’ She stopped, and she and Luc exchanged a glance, and a little smile that crinkled the tanned skin at the corner of his mouth. ‘My stepbrother, I guess? Well, anyway. Here we all are. Don’t just stand there, Luc.’

Luc smiled again, then he ducked his head, awkwardly, and moved backwards into the room, making space for the rest of us.

‘Can I get you a drink?’ he said as we filtered past, Fatima and I tongue-tied by the unexpected presence of a stranger, and a strange boy at that, when we’d been shut up for so many weeks with only other girls.

‘What have you got?’ I asked.

‘Wine,’ he said with a shrug, ‘Côtes du Rhône,’ and suddenly I knew what that accent was, what I should have realised from his name. Luc was French.

‘Wine is good,’ I said. ‘Thanks.’ And I took the glass he gave me and knocked it recklessly back.

It was late, and we were drunk and limp with alcohol and laughing and dancing to the records Kate had put on the turntable, when there was the sound of the door handle, and all our heads turned to see Ambrose coming through the door, his hat in his hand.

Fatima and I both froze, but Kate only stumbled across the room, tripping drunkenly over the rug and laughing as her father caught her and kissed her on both cheeks.

‘Daddy, you won’t tell, will you?’

‘Get me a drink,’ he said, throwing his hat on the table, and ruffling Luc’s hair, where he lay sprawled across the sofa, ‘And I never saw you.’

But he did, of course. And it’s his own sketch that gives him the lie, the little dashed-off pencil thing that hangs at the crook of the landing, outside Kate’s old bedroom. It’s a sketch of the sofa, that very first night, with Luc, and Thea and me tangled together like a litter of puppies, arms around one another, limbs entwined until it was hard to tell where my flesh ended and Thea or Luc’s began. Perched on the arm of the sofa is Fatima, her bare legs acting as a chair back for Thea to lean on. And at our feet is Kate, her spine against the battered couch, her knees to her chin, and her eyes on the fire. There is a glass of wine in her hand, and my fingers are laced in her hair.

It was the first night that we lay and drank and laughed, curled in one another’s arms, the stove flames warm on our faces, heating us through, along with the wine – but it was not the last. Again and again we would come back, across fields crunchy with hoar frost, or past meadows full of baby lambs, drawn again and again, like moths to a flame that shone through the darkness of the marshes, drawing us in. And then back through the pale spring dawns, to sit heavy-eyed in French, or wending our slow laughing way through the marshes on a summer morning, salt water dried into our hair.

We didn’t always break out. After the first two weeks of each term, the weekends were ‘open’, which meant that we were free to go home, or to friends, provided our parents gave permission. Home wasn’t an option for Fatima or me, with my father permanently with my mum at the hospital, and her parents away in Pakistan. And Thea … well, I never enquired about Thea, but it was plain that there was something very wrong, something that meant that she either could not or would not go back to her parents.

But there was nothing in the rules to say that we could not accompany Kate, and we did, most often packing up our bags and walking across the marshes with her on Friday nights after prep, returning Sunday night for registration.

At first it was the odd weekend … then it became many … and then at last most, until Ambrose’s studio was littered with sketches of the four of us, until the Mill was as familiar to me as the little room I shared with Fatima, more familiar even, until my feet knew the paths of the marsh by heart, almost as well as Kate.

‘Mr Atagon must be a saint,’ said Miss Weatherby, my house-mistress, with a slightly thin smile, as I signed out yet again with Kate on a Friday night. ‘Teaching you girls all week, and then boarding you for free all weekend. Are you sure your father is OK with this, Kate?’

‘He’s fine,’ Kate said firmly. ‘He’s more than happy for me to have friends back.’

‘And my dad’s given permission,’ I put in. With alacrity, in fact – my father was so relieved that I was enjoying myself at Salten, not adding to his worries by clamouring to come home, that he would have signed a pact with the devil himself. A stack of pre-authorised exeat forms, by comparison, was nothing.