The collaboration we want: Simone Rocha x J Brand

Simone Rocha's Dublin accent - every bit as coaxingly conspiratorial as that of the shrink Gabriel Byrne played in In Treatment - echoes around her elegant, canal-side studio in Haggerston, east London as she leads me from her art book-lined inner sanctum to the open-plan outer office with The Rail - the one with her about-to-launch collection for J Brand hanging from it. "One of the first questions the people at J Brand asked me," she burbles, "was what style of jeans I liked - and well, I wasn't going to lie. So I told them I didn't have any."

The scrupulously honest response clearly didn't deter the J Brand team. On November 14, 10 months on from that first meeting, Simone Rocha's 20-piece collection lands exclusively at Dover Street Market; later in the month at Net-A-Porter; and on December 1 at Selfridges - and after that, other stockists. From those two above-the-title credits alone, we may deduce that this is a hotly anticipated collaboration. Examining the rail, it's not hard to see why. Somehow, despite never wearing the stuff herself, Rocha has alchemized her own ethereally-embellished Edwardian aesthetic into the most robust, democratic fabric on the planet, albeit not in blue denim but in her beloved black, Pepto Bismol pink and a perfect red: "not too blue, not too orange - I wanted a red that would work on every one".

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These are colours that have recurred in her own collections, and they're not the only element fans will recognise. There are those familiar Rocha trousers, with ruffles at the hips or on the pockets; oversized bombers with ruffles on the sleeves; and a simple ruffle-trimmed pinafore dress that can be worn on its own, or over the ruffled trousers. "You've managed to make jeans look like bloomers," I joke. She laughs (she's a giggler). "Oooh I love bloomers. I often wear them."

Of course she does. She gets hers in markets, especially the one near her parents' house in the south of France, and with her lilting laugh, invariably charms the stall holders into throwing in an extra pair.

Today she's wearing a pearl-embellished black knit from last season's collection, a ¾ length, black 'pervert's skirt' ("it's got sheer mesh sides, see?") and pearl embellished ballet flats. It is part Duchess of Devonshire, part Charlotte Rampling in The Night Porter. It is also her uniform. "Ooh I love uniforms. I even liked the brown one I had to wear to school in Dublin," she says, showing me the picture, pinned next to her desk, of her head, and those of her team superimposed onto some old fashioned school uniforms. Pupils only wore strict uniform for the first four years at Alexandra College, the private girls' school she attended in Dublin before art school and Central Saint Martins.

"I was so disappointed when we could start wearing our own clothes that I invented my own version. It was basically the same as I wore the first four years. But in black - black skirt, black jumper and knee socks." In essence, it's what she's wearing today, albeit an upgraded version.

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She is, by her own admission, a period-drama nut. "Downton, North and South, and all the really rubbish ones too, I love them. I can watch them over and over. I love history, I love regalia…."

So far, so consistent with the daughter of John Rocha, the half-Hong-Kong-Chinese, half-Portuguese, much-loved Dublin-based designer who for 30 years charmed London Fashion Week with his own spin on historical romance. Black and ivory were his favourite colours too. Wasn't she ever tempted to rebel?

"If you ask my parents, from the age of 15 I was wild ," she laughs. Her teen spirit seems to have consisted mainly of peroxiding her hair and partying. Even then she never wore jeans very much.

And that's precisely what made this project so interesting. "I loved the idea of working with such an honest, authentic fabric." For her own collection, Rocha normally works with pricy tulles and wools and bonded silks, trying to make everything stiff enough to hold a really bold constructed shape, but also soft and light. "Denim," she reports, "was a real challenge."

The J Brand team, being aficonados, naturally showed her scores of different denim weights, washes and treatments. She selected just one, which was then dyed in batches. The results are pretty as a Downton tableau but also eligible for being worn in a more minimalist, androgynous way. It strikes me that what's she has done is make a denim collection for people who don't wear denim. More giggles. "Good," she says, "because I've absolutely got my eye on this oversized denim bomber."