Katy Guest: How to be Posh? Eat toast and wear wood stain

You've got to hand it to Victoria Beckham: she always knew her own limitations. Even in the glorious girl-power heyday of the Spice Girls, she admitted she was a rubbish singer. Launching a perfume range at the Palazzo Pisani Moretta in Venice in a Fifties-style petticoat and eye-popping corset, she announced: "I look like one of those dolls that sit on top of your toilet roll." She has always seemed as if she can't believe her luck. Which is why you have to worry about her putting herself forward as the Nigella Lawson of the tulle skirt.

It is unfortunate for Posh that her new style manual, That Extra Half an Inch: Hair, Heels and Everything in Between, was released in a week when we are told that some anorexics see her as a style icon. On a pro-anorexia website - the sort of place that tells you exactly what you can do with those extra half inches - a 14-year-old raves: "She is so gorgeous and skinny... she has beautiful bones sticking out of her chest."

No one is saying that Victoria Beckham is to blame for anorexia, or has it. But it is time that she was honest about the Sisyphean efforts that go into staying thin. Anyone over the age of 17 who claims she is clinically incapable of gaining weight is either deluding herself or suffering from an embarrassing tapeworm. And Victoria's tapeworm long ago contacted the National Union of Intestinal Parasites demanding a delivery of cheesy nachos and a helicopter out of there.

"That Extra Half an Inch is a revealing and practical book," insists the blurb, comically. "Whether you're getting ready for work, a night out on the town or even doing the school run, this is Victoria's personal guide to feeling confident and looking great every time you step out of the front door." Provided the door jamb is wider than you are, Donatella Versace is your personal dresser and you think Cuprinol is a moisturiser.

Readers of the book may be surprised by her description of life chez Beckham. This crazy idea we all have that she "staggers round all day in couture and stilettos" is, she laughs, "as mad as thinking you will survive on caviar and champagne". Now regular Posh-watchers will know that she supposedly survives on sushi and pineapple, but that's not true either, apparently. "Toast is my favourite food," she reveals. Of course, that is special anti-gravity toast that burns calories as you eat it and swells up inside your bosoms, but you get the picture. The most unnerving bits are Victoria's style rules. "When you're having a bloated day," she advises (for instance when you have accidentally walked past a chip shop and ballooned all the way up to a size four), "always have some emergency figure flatterers." For some of us, only getting stuck in a Corby trouser press would do the job.

"Don't let it all hang out - leave a little to the imagination." Leave what to the imagination? The name of your cosmetic surgeon? "Multitask, planning outfits while you're doing something else - getting ready for bed or taking a bath," she says, brilliantly. What, thinking and bathing at the same time? How dangerously revolutionary. "If you can't afford the real thing, go to St Tropez in a can." And if you can't afford that, go to Blackpool and roll in tar?

"Always wear great underwear - even as outerwear if you're brave enough." And she is saying this to women in Britain? In October?

Bless Posh, we truly love her crazy outfits, cast-iron pout and unshakeable insistence that the sensuous curves in her publicity photos are achieved simply by "really sticking my bum out". But which of us lives her life - or would want to? It's all very well telling girls how to dress for the red carpet, but we liked Victoria much better when she dressed for the Harlow Odeon.