There’s a touching moment in the documentary The September Issue, which tracks the making of a bumper edition of American Vogue in 2007. The creative director Grace Coddington sits at her desk – tired, nearly teary and twiddling some mesclun salad with a fork. Part of an epic fashion shoot has been dropped by Coddington’s boss, the formidable editor-in-chief, Anna Wintour.

The unexpected star of The September Issue, Coddington – flame-haired, sprightly, wickedly humoured and supremely driven – looks crestfallen. Her Brassaï-inspired shoot has taken months in preparation. Just for the shoe call-in alone, the film reveals not one but a handful of assistants taking notes on Coddington’s vision and the Parisian turn-of- the-century T-bar style shoes she’s looking for.

The shoot, with photographer Steven Meisel, would take place, not in a smoke-stained Pigalle brasserie, but in Keith McNally’s authentic-looking, if not behaving, brasserie Balthazar in SoHo, New York. But magical transformation, make-believe and romance are what Coddington’s vision is all about – a vision that has made this Welsh-born former model one of the world’s leading, most influential stylists. Talking to camera, Coddington confesses her frustration at the culling of a beautiful opening image: “I care very much about what I do… It gets harder to see it just thrown out. And it’s very hard to go on to the next thing.”

Coddington and Wintour dominate this film with their fascinating, tension-filled working relationship of more than 20 years’ standing. Wintour stands as the apparently unemotional editor-in-chief – a woman who makes decisions at lightning speed, wields vast budgets, directs designers and spots young talent.

Coddington, by contrast, is a diehard romantic with an intensely lyrical, highly crafted and frequently witty vision that translates on to pages the fantasies and creations of designers via her own dreams. Her visual language spans from graphic shoots, in which you want to buy the dress, the socks, the heels from the page, to epic voyages that leave you thinking, “I wish I could dream like that.”

Coddington was born to hotelier parents on the island of Anglesey, Wales, in 1941 – a million miles away from both the perfumed salons of couture and the ruthless and highly competitive business of the fashion business. A gangly, nervy, pale-skinned schoolgirl, convent-educated, she subscribed to Vogue and ran up her own outfits on a Singer sewing machine. She moved to London, enrolled at modelling school (at that point “teaching” involved learning etiquette, even how to curtsy), but her big break was winning the Vogue model competition.

Today, stylists are integral to the image-making of fashion. But back then the model (hauling around a bag of clothes, girdles and wigs) and the photographer were the image producers. Old photographs of Coddington in British Vogue show the long-limbed, ethereal-looking beauty showing off the designs of the day. Coddington recounts those years with a warmth and still rather wide-eyed surprise at how she, the shy Welsh girl, made it, before a horrendous car crash put a stop to her burgeoning career. The incident sliced open her eyelid, and the scarring can still be seen in slight relief.

So at the age of 28 she was interviewed by British Vogue’s editor, Beatrix Miller, and was employed as a junior fashion editor. She worked with the greats – Helmut Newton, David Bailey, Norman Parkinson, Sarah Moon – creating daring, ground-breaking shoots, which were weeks in preparation and days in actual shooting. Like working on the set of a film, the crew became characters living out a narrative.

Coddington then joined American Vogue to work under Anna Wintour in July 1988, substantially upgrading her income and style of living – although The September Issue reveals she is happiest with her cats, an unpretentious minimalist, monochrome wardrobe, her partner Didier Malige and a pile of books.

So Wintour and Coddington have shared the best part of a working lifetime together. In The September Issue, we see their shorthand banter, the facial expressions which often say more than words, and we see both thinking on their feet. When Wintour kills a shoot about the colour blocking trend that had been assigned to the browbeaten contributing editor Edward Enninful, Coddington is empathetic, but warns, “You’ve got to be tougher… Don’t be nice. Even to me. Honestly, because you’ll lose.” And just before the September issue goes to press, we see Coddington, less crestfallen, reshooting that colour blocking story on Wintour’s orders, turning it around in a day. The editorial is a vision of dynamic fashion: the model jumps, laughs, plays. Just as ideas are running thin, Coddington pulls in the documentary’s cameraman, Bob Richman, asking him to jump into shot, his belly popping out in a rotund mound next to the svelte model. It reverberates with the Sixties – with David Hemmings in Blow-Up, the graphic style, the exuberance – and it’s a true winner.

The September Issue is out on September 11

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