Catwalk confidential: A life in the day of Britain's hottest models

What does Agyness Deyn eat? (Not much.) How does a size zero survive London Fashion Week? Susie Mesure gains unprecedented access behind the closed doors of high fashion

Backstage all is chaos. Models scheduled to turn up two hours ago for some primping and preening are only just arriving. Back-to-back castings mean everyone is running late and tensions are soaring. Time is running out before the girls are due on the catwalk.

Such was the scene just hours before the Topshop show was due to start on Friday afternoon, but the same frenzied picture could describe any of the 54 shows that will take place during the course of London Fashion Week, which kicked off yesterday with collections from Paul Costelloe and Ben de Lisi.

"It's mental, mental chicken oriental," said Fiona Ellis, who scouts for new faces at Independent Models, formerly ICM Models, of the fashion extravaganza, which will have generated more than £100m-worth of business for the capital by the time Stella McCartney wraps up the event with her sporting range for Adidas on Thursday night.

Imagine, then, the turmoil that would have ensued had the British Fashion Council's landmark ruling on the health of models decided to add a mass weigh-in to the pre-show angst that designers have to endure. Weighing models was among the proposals put to the panel conducting an inquiry into models' health, which opted not to ban stick-thin girls but instead to recommend that under-16s be kept off the catwalks.

Yet still the disarray backstage at catwalk shows is legendary. As the fashion show proper kicked off yesterday with Costelloe's show at the Natural History Museum, teams of make-up artists pounced to ensure each girl was catwalk-fresh within minutes of her arrival. The Dublin-born designer's collection mixed neutral colours with bold, colourful floral fabrics.

As the shows progress – and this week will see works by designers Matthew Williamson, Julien Macdonald and Luella Bartley – the pressure rises. The models are increasingly late as shows overrun.

Elbows fly and tempers boil as too many people crowd into too little space to do battle with impossibly delicate creations. Each girl will often have to change several times per show and, once it's curtain's up, the clock is ticking.

There's little time for food and drink. In terms of sustenance, mini-bottles of champagne are a perennial favourite – although this year agencies have ordered designers to make sure there is also water and plenty of food available. Food at some of the shows, such as Paul Smith's, is so good that backstage passes are at a premium. Not that this means the girls will necessarily touch it, of course.

Telling designers that "establishing a healthy backstage environment is a priority ... with good-quality food" laid on was recommendation six of the Model Health Inquiry's 14 points. Making sure the venue is "demonstrably drug-free" might prove a harder task, with the use of cocaine a renowned way for girls to pep themselves up between shows and keep their hunger at bay.

Food, drink and a "just say no" policy will only go so far to reduce the pressure on models desperate to become the next big thing on the catwalk.

On Friday morning, even as the BFC was trumpeting its safeguards for models, Olivia Dunin, one of the girls it is trying to protect, threw herself into the maelstrom that is the run-up to Fashion Week, squeezing in up to 12 castings a day, hitting the road before 9am and working into the evening. "Normal modelling days involve around three castings a day, but this is London Fashion Week, so I can be going to 10 or 12 when it's like this," she said. "I get going about 9am and I don't usually finish till about 6.30pm."

The first casting was at an anonymous-looking place in south London's Borough Market which turned out to be someone's living room. It was being used by Danielle Scutt, a designer showcasing bondage swimwear.

By the time she had walked the length of the living room it was clear she was not what they are looking for. Olivia was "too pretty" for the job, the casting director confided; they were looking for someone with a more masculine look. Less than five minutes later she was on the road again. "That was a weird casting," she said. "There are usually lots of girls waiting to be seen and it's usually in a studio rather than someone's house. I'm not too bothered though, you can tell straight away when you're not what people are looking for."

The next casting was for Unconditional, the label launched four years ago by Philip Stephens. She was desperate to be picked. "All the eyes, including the casting directors', are on you and, if you get it right, the campaigns follow," she said. The shy teenager was transformed into a devastatingly beautiful woman, every inch the professional model, in a white trouser suit and heels.

The slim-but-not-skinny gap year student (she is taking a year out after scoring two As and a B at A level before studying architecture at Nottingham University) thought the ban on under-16s was the right step for the industry to take. She was not perturbed by stories about unhealthy models, because "you can tell just by looking at a girl if they are doing themselves harm to get into fashion".

And there's the rub: too many girls have already done themselves harm for fashion. Publicly, but also behind the scenes, the industry is keen to escape the role of bad boy in the size-zero drama that has troubled fashion watchers since the notion of an improbably small dress first made its way across the Atlantic from the US.

So far this has comprised little more than an insistence that all their garments are made in a sample size 10 (albeit not a size 10 that most women in the country would recognise). Yet evidence of changing attitudes is emerging. The agency Models One told a "size zero" girl last week that she needed to put on a stone before they would let her work. And Ellis, at Independent Models, said that last summer she sent away a Polish girl who had worked for her for three years because she'd shed too much weight.

Caryn Franklin, a fashion writer, sees a move away from the "very young and thin" aesthetic that has predominated ever since Balenciaga kickstarted the trend by working the contrast between his voluminous creations and the ultra slim.

After that, lazy designers saw thin as a "short cut to edgy", Franklin adds, but now, "with the intellectual move away from very thin, designers are focusing on the criticisms".

Lee Lapthorne, who runs the fringe On/Off event that runs alongside fashion week, says: "The aesthetic is starting to change. Designers and stylists are searching for more healthy-looking models."

Sarah Leon, head of new faces at Select Models, adds: "There is a movement towards a glamourous, womanly, almost film-star type of look." She ascribes the new aesthetic partly to the fact that actresses are now such a "huge part of fashion, hogging the big campaigns, so to look like them helps", and partly to the "publicity surrounding the 'real body' shape issue".

Debbie Jones, a booker at Models One, says: "I've had one show that specifically asked me for girls who are very tall, curvy and womanly with fit bodies. One girl is a 32D."

And certainly demand is soaring for girls who are not your usual stick insect. Daisy Lowe, the 18-year-old daughter of Pearl Lowe and Gavin Rossdale, the Bush frontman, is in "huge demand" for this week's shows, her agent tells me, despite having something approaching a figure.

The new crop of models is best represented by Agyness Deyn, the peroxide, pixie-cropped girl from Hull who used to work in a fish and chip shop. Best mates with Henry Holland, the hot young designer who brought us this summer's slogan T-shirts, Deyn embodies the individual look that agents say is "very now". She was flat out during last week's New York shows and will be similarly pushed this week.

The fashion industry will be crossing its fingers that there isn't another tragedy similar to that which claimed the life of catwalk model Ana Carolina Reston, a Brazilian whose death from anorexia shocked the world and turned public attention on to the chaos backstage.

Additional reporting by Ian Griggs