Trinny Woodall and Susannah Constantine: In bed with the first ladies of fashion

I phone my friends (friends? Ha!) at The Independent office and ask them to get back to me with a single sentence that, for all of them, sums up my fashion sense and style. Don't worry about being hurtful, I say. I won't mind. I can take it. The reply, when it arrives, goes: "A non-directional style featuring brave elastication and very cheap clothes". Good job I can laugh at myself. Or it would be, if I could. Still, I only cried for four days and was pretty much back to normal, after I'd had my stomach pumped. Then I phone my mum. My mummy loves me. My mummy will say something nice. "Mum, how would you describe my style?" She thinks momentarily. Then says: "Well, darling, let me see. Non-existent?"

I'm not sure the hospital had expected to see me back again so soon, frankly.

Whatever, time for fashion journalists Trinny Woodall and Susannah Constantine, no? Trinny and Susannah do What Not To Wear, the annoyingly compulsive BBC2 fashion makeover show. Annoyingly compulsive? Look at me, carrying on as if I watch it in spite of myself, as if I should know better. The real reason I watch it? Because I love it. Because it's just so squirmingly enjoyable. But Trinny and Susannah? Scary or what? Posh or what? Rude or what?

This is how the show works: a "victim", nominated by particularly sadistic, mean friends (hey, I know what it's like to have a few of those!) is secretly filmed for a week wearing her usual, pitiful, Asda-going wardrobe, so that Trinny and Susannah can later sprawl on their lovely white sofa, watching the footage, and saying things like: "She shouldn't be wearing vest tops with those upper- arms." And: "Look at her VPL. Those are knickers we have to ban." And: "Those trousers make her arse look huge." And: "Look at her breasts. They're like torpedoes. I thought it was someone's head."

Basically, it is ego-annihilation at its most thrilling. "I feel like a bucket of poo," said Sam Blower, a 30-year-old secretary, after being talked through the footage and shown the evidence of her own, startling VPL. But then, the person is given loads of advice – "Look at that great lump going across your chest... it's like you've lost respect for your bosom... go for a bra fitting" – and £2,000 to spend on a brand new wardrobe. The results? Pretty spectacular, even if Trinny and Susannah do say so themselves. "She looks fab," they'll say, "for someone so common." Actually, they don't say the last bit, although I sometimes suspect that they might rather like to. I know people who would quite like to give them both a good slap. And I think it's this that, possibly, does it.

We meet at 10am, at Joe's Café, which is on Sloane Street, next to Gucci and down the road from Prada and Dior and Armani and all the other shops that aren't Whatever She Wants, a favourite of mine on the Holloway Road that offers much brave elastication at extremely good prices. (I don't know how they do it). I am very nervous. Help, I am thinking, they're going to eat me alive and then spit out my bones from which, likely as not, they'll make themselves some bargain-priced, Agent Provocateur-style corsets. Help, I'm thinking, I know nothing about fashion. Although, that said, I did once meet Jasper Conran. "Jasper," I said. "Pockets this season? In or out?" Strangely, I wasn't immediately promoted to a front row at all the catwalk shows from then on.

Susannah has already arrived. Susannah is wearing a Marc Jacobs, puff-shouldered denim jacket and long grey skirt from Topshop. She is very pretty, in a very English way, with blonde hair, a clear, pale complexion, very blue eyes. "I think you'll agree you have your work cut out," I say, by way of introduction. She is marvellously comforting. She has two small children (Joe, three, and Esme, 10 months) and points out the spilt baby milk on her boots. I say, that's nothing. I say, I'll be walking along when I'll suddenly feel yesterday's sock falling out of my trouser leg.

"OH, but it's worse, much worse, when yesterday's knickers drop out." "And that happens to you, Susannah Constantine, one-time habituée of Tatler's society pages? Yesterday's knickers fall out of your trouser leg?"

"Absolutely," she says. "Are you big knickers or little knickers?"

"Um... sort of standard, greyish knickers from M&S," I say.

"Get big knickers from Knickerbox. No VPL."

VPL is an outrageous crime, obviously. Perhaps, even, there should be a national register of offenders.

Ah, here is Trinny. Trinny is pretty, too, but skinnier, bonier, edgier, with more angles, less bosom. They both love talking bosom, though. "What size are your tits?" asks Trinny. I say I think I'm a hopeless 34A. Trinny is alarmingly direct. Trinny leans across the table, has a bit of a feel. "Nope," she says. "You're a 32B." Trinny is wearing checked trousers from Zara. Trinny, though, isn't Trinny at all, actually. She is Sarah Jane. Trinny became Trinny when, at school one day, she decided to cut off the plaits of a girl she hated. Well, only the one plait. She only got that far before she was collared and dispatched home. Now, as it happens, her father, a banker, was friends was Ronald Searle, the illustrator who created the girls of St Trinian's, and on that particular day, Ronald was visiting. "You're just like a St Trinian's girl," said Ronald. So, from then on, she was Trinny.

Trinny is much more fastidious about her clothes than Susannah or I. "When I'm stressed, I'll refile my jumpers in order of colour. Have you seen the new Nike shoes? They're criss-crossed and so GORGEOUS."

What, they ask, do I want from today? I say I don't know. I say I only know what I don't want, which is:

Anything that needs dry-cleaning. It'll either never be taken, or taken and never picked up.

I do not own an iron.

No short skirts, as my legs could happily swing in the window of a German delicatessen.

No tights. I hate tights.

No heels. No pointy toes...

"Hang on," interrupts Trinny. She and Susannah confer. No, they decide. We won't do Prada or Gucci or Armani – and all the other shops that I have never had the courage to go into – as they had originally planned. We'll do Topshop instead. I am much, much more Topshop, they agree.

Should I be offended? NO, they say, because, as it happens, they are more Topshop, too. They're as frightened of the super-chic shops as anyone. "Gucci is the worst," says Susannah. "The doormen? What's all that about?" Trinny says that she has a friend who went into Kenzo, asked for something, and was told: "We don't have it in your size and, even if we did, it really wouldn't suit you."

First, though, would I mind if they made me an appointment with their hairdresser, Roger at Richard Ward on Sloane Street? I think their view is that the Noddy Holder look isn't exactly in, and possibly never has been.

So, off we go in a taxi to Topshop at Oxford Circus. I am excited now. Whoever said appearance doesn't matter was bonkers. Indeed, I'm sure if, over the years, I'd eased up on the elastication and done less accidental sock-shedding, I'd be chairman of ICI by now. Who else dresses badly? Who else would they really like to get their hands on?

"Dani Behr and Alex Kingston," says Trinny. "I'd love to take some scissors to Stella McCartney's hair," says Susannah. The worst fashion crimes, apart from VPL? "Tapered trousers," says Trinny. "They accentuate the widest part of the body. Marks &Spencer is the worst perpetrator." Susannah adds: "And leggings, obviously."

Is there something that couldn't be revived if a great fashion house starting doing it? What about the shell suit? What if Armani started doing shell suits? Susannah thinks that someone is doing the shell suit. In velour. And? "It's very nice."

Trinny disagrees. "Susannah, are you mad? It's hideous!"

Trinny and Susannah first met at a dinner party, given, it turns out, by Viscount Linley, whom Susannah famously used to date. I say "famously" because, even though it was all of 15 years ago, she is still referred to as "Viscount Linley's former girlfriend". Irritating?

"I think, actually, I'm getting beyond it now. Thank you very much for reminding me." "Just think, though," I say. "If you'd married him, you could have a marquetry table for every room."

"And they don't come cheap," adds Trinny.

We all giggle. We are getting on quite famously now.

They didn't really talk to each other at that dinner party. "We were like dogs," says Trinny. "Sniffing round each other." They were later reintroduced by another friend, who suggested they work together, as they had a mutual interest in style. This was followed by a regular "Ready To Wear" column in The Daily Telegraph, which, in turn, was followed by a Ready2Wear website (now, sadly, gone the way of most dot.coms) and a hopelessly cheap satellite telly programme. "It was shot in our homes, with dogs as models," says Trinny.

Both are now married. Susannah to Sten Bertelsen, a Danish entrepreneur, and Trinny to Johnny Elichaoff, a financial adviser who used to manage bands like Breathe and Fairground Attraction. I wonder if they share their wives' passion for style? Sten, says Susannah, is pretty useless. "I once dressed in spots, stripes and flowers, with a Mary Hopkin hairdo to see if he'd notice, and he didn't.

Johnny is much better. Johnny at least tries to be trendy. "Yes," says Trinny, "but you should see his yellow trousers..."

Into Topshop we go. Now, this is the thing about Trinny and Susannah. If I was to come in here on my own, I'd think: "I'm much too old for this cheap crap" and directly exit again. But Trinny and Susannah know what isn't crap. Trinny and Susannah know what to look for. Trinny and Susannah truly have an eye for such things. They have no grey areas, unlike me, who has lots (especially in the knicker department). Trinny and Susannah elbow all the 13-year-old girls looking at things in pink out of the way, and go to it, whizzing off like those hard, steel balls in a pinball machine, bouncing off the rails, gathering up Marc Jacobs puffed-sleeve rip-offs, Moto jeans (which, they claim, are the best jeans ever), long skirts cut on the bias (whatever that means), lots of dangly accessory things, a wild burgundy Afghan-style coat.

They are deliciously bossy. I express interest in a sheepskin coat. "Ghastly," says Trinny. "So Del Boy," says Susannah. "I wasn't serious," I lie, quickly. We spend ages trying things on. A good tip? If you're a short pear-shape, they say, go for hipsters. They make your bum look smaller. I buy bag-loads of stuff.

Another taxi ride, to the hairdresser. This time, though, we don't talk velour shell-suits. This time, we talk books. We talk Ian McEwan's Atonement and Patrick Süskind's Perfume and William Boyd, Ayn Rand, William Trevor, Jane Austen. They are both voracious readers, although you hadn't expected them to be. Why? Because, I guess, you had them down as the sort of posh It-girls who, by rights, should have become chalet girls or silly, ghosted magazine columnists, if they'd bothered to work at all. This drives Susannah mad. Someone, she says, once wrote an article about her that said she'd never done a day's work in her life. "I'm not kidding, it made me cry for a year. I've always worked," she says.

And Trinny, even, used to work in the city, in commodities, which she'd still be doing, if she hadn't met Susannah. She is glad she met Susannah. What would she be like if she was still in commodities? "Unhappy and depressed," she says.

True, they come from wealthy backgrounds. Trinny's father was a banker, as I've said, while Susannah's family, who came over with the Conqueror, made their money in property and shipping. But wretched things can happen to rich people, too. Both, for example, were sent to boarding schools, which they hated. Trinny was sent at six. Six! And she has only lately connected her fear of being naked (in front of anyone) to the time when she was caught having a water fight in the dorm, and made to stand naked by the gong in the hall, while all the other girls filed past for breakfast.

Susannah was dispatched at 11, and remembers her first night away from home clearly. "I sobbed uncontrollably into my pillow." Later, she says, she stole her older sister's teeth braces – the ones she'd grown out of – to wear at night, "because it was the one thing that comforted me". There is a kind of vulnerability to them both, which, after a while, makes them considerably more huggable than slappable.

We arrive at Richard Ward, where I am given a good Roger-ing, so to speak. Roger is very nice, and doesn't tut-tut too much over my fringe, which is kept at a manageable length by singeing it every time I light a fag off the gas cooker. Then we part, with much hugging all round. I wear one of my new outfits home. I wear burgundy cord hipsters, a brown zip-up top thing, brown chunky boots and the wild Afghan-style coat that, Trinny and Susannah insist, isn't too toy-poodle at all. It all makes me very happy. In my head, I'm singing: "I'm so pretty, so pretty and witty and gay (although in the old sense of the word, not the new, because I'm not sure I liked Trinny feeling my breast at all)".

I go via the office. Well, I ask, what do you all think? A staggering improvement, it's decided. I get home. Well, I ask my son, what do you think? "You're not taking me to school in that coat," he says. "You can park round the corner and just drop me off."

I'm expecting the ambulance at any minute. Do you think the paramedics know me by name now?

'What Not To Wear' is broadcast on BBC2 on Wednesdays at 8.30pm