In the bag it looked like one of those horrendous "slinky scarfs" from the Seventies. Unfolded, it became a long, thin sack - an elegant sack, but a sack nevertheless. The label said 100 per cent polyester, size eight. It was a black Issey Miyake tube dress, price £200, the sort of over-priced, high-concept construction that looks fine on Kate Moss but on a 30-year-old, 5ft 7in, 101/2-stone journalist with a bad case of pre-menstrual water retention? I think not.

Still, a commission is a commission and I took it home. The good news was that I could squash it into my backpack, along with my (never used) swimming gear, Filofax, bike pump, spare inner tube (very glamorous) and cycle home as usual. Ideal for travelling, I thought idly. Better still, no ironing (my personal grooming phobia). I hung it in the cupboard and resolved to lose weight.

A week later, having lost not a pound, I decided to take the plunge and wear it at our housewarming party. If any guests started sniggering at the sight of a small whale bulging out of a giant condom, I could rush upstairs and change into a sober, unrevealing pinstripe suit. Meanwhile, up went the hair to make me look taller, on went the cellulite-gripping tights, the painfully high black shoes, the red lipstick ... and the dress.

It slipped on like a dream: no buttons, no zips, no tight digging-in bits. It felt, disconcertingly, like I wasn't wearing anything (except for the viciously tight tights and shoes), somehow clinging to the good bits (breasts, legs) and gliding over the bad (stomach). Miraculously, it was just the right length (long). I felt like, well, if not a million dollars, then at least £200. And for the first time since puberty, I was size eight! Oh, fabulous day.

Exhausted by all that self-admiration, I slumped on the sofa to wait for the guests. My husband appeared and looked doubtful. "Mm," he said. "It's ... great." Just before launching into a full-scale pre-party bickering session, I glanced down. Disaster! The dress had rucked up and developed a spare tyre! I stood up, relieved that this was going to be a standing-up party. The tyre disappeared.

The doorbell rang, I glided downstairs (going up was liable to trip me up, given the length of the dress). "Darling!" said my friends, suddenly turning into extras from Absolutely Fabulous. "You look amazing! Is that Fortuny?" "Miyake," I replied airily,suddenly unsure whether one's favourite designers are referred to by their first or last names.

And so the evening progressed. The dress did not become limp or crumpled or stained or uncomfy or embarrassing. It even seemed immune to mulled wine slops. The guests continued to marvel, as they never have before. (True, I am often to be found in faded leggings and baggy T-shirts, but I include in this reckoning my wedding day, when it is obligatory to compliment the bride). I didn't sit down. The new house didn't get a look-in.