Diane Warren: She writes the songs

She swears like a trooper, works in a tatty office and has a parrot as her closest friend. She's also the writer Britney, Christina, the Sugarbabes and Cher come to when they want a lurve-song hit. Mark Ellwood meets Diane Warren, the woman with a track record others can only dream of

Though few outside the music business know her name, Warren is the high priestess of heartbreak (despite the fact, as she freely admits, that it's a decade or more since she last had a proper boyfriend). If you've fallen in love or had your heart broken in the past 20 years, chances are it was to a soundtrack of her songs: Warren's written more than 1,000, but among the best known are Toni Braxton's "Un-break My Heart", Celine Dion's "Because You Loved Me", Aerosmith's "I Don't Want To Miss A Thing" and "How Do I Live" by LeAnn Rimes. On the way, she's picked up a Grammy, been nominated for six Oscars and, of course, made millions of dollars.

Yet Warren's publishing company, Realsongs, is located in a nondescript building at the less-glamorous end of LA's Sunset Boulevard; it's not far from the famous Capitol Records HQ, built to look like a stack of 45s. The gleaming offices, walls crammed with gold records, are beige and bland - oddly ordinary for a woman famous in the music business for her eccentricity and strangeness.

Before meeting Warren, I'd been warned not to expect a gushing, poetry-spouting sweetheart; instead, think a Beavis and Butt-Head-type who favours four-letter words that aren't "love". "She's liable to eff and blind, and come out with words that would make men blush, let alone women," laughs EMI UK's chairman Peter Reichardt, who's known Warren for almost 20 years and handles her songs outside the US.

"I've heard her on the phone to record company chairmen saying, 'You're talking out of your ass, of course it'll be a fucking hit' and then just putting the phone down." But aside from a necklace that bears the words "fuck you" and a grumpy pet parrot called Buttwings (more on that later), in person Warren proves softly spoken, shy and rather charming. Hiding behind a shaggy fringe, she talks quietly for more than an hour; the only shift comes when discussing music (then, her voice shows a steely determination, clear evidence of what her close friend Simon Cowell calls her "weird combination of arrogance and crippling insecurity").

Diane Eve Warren was born the youngest of three sisters n 1956 in southern California, and grew up in a suburban home listening avidly to the radio. By the time she was 11, Warren had started writing songs; three years later, she persuaded her salesman father to start shopping them round with her to publishers. "When I got to about 14, I got passionately crazy about songwriting... I lived for it," Warren explains, adding, "I don't remember what my exact first song was called, but they were really bad, although at the time I thought they were good."

Despite her determination, she didn't snag a deal as a teenager but, set on being a songwriter, she spent the next 10 years trying to * get noticed. "I tried to pull people over in their cars when I recognised them, or I'd follow people into bathrooms," she chuckles. "It was pretty - what's the word? - persistent." Even today, Warren is known for her tenacity - it's not unusual for her to badger performers and producers personally until they cave in and commit to recording a track or two. "No one sells a song as well as Diane," Peter Reichardt explains. "If she believes in something, boy, does she believe in it."

Eventually, Warren took a job as a messenger for a record company ("I delivered shit to the wrong address," she says dismissively). This allowed her to drop off her own packages to the powers that be alongside regular envelopes; she lasted two weeks before being fired. But Warren did finally find a publisher and start to sell songs - her first big hit was DeBarge's "Rhythm of the Night" in the mid 1980s, a cheery, up-tempo song that's very different from the big, weepy ballads she's known for now. But that was just the beginning, since then she's been churning out multi-platinum number-one songs for artists as varied as Michael Bolton, the Cult, Whitney Houston and the Sugababes.

On average, Warren works on one song each week, and is usually in her office every day but Sunday for at least 12 hours. "I can't work on more than one song at a time, it's too schizophrenic," she says. "I put everything, my heart and soul, into what I'm doing." A relentless perfectionist, Warren has spent the entire day before we meet tweaking two lines until she is satisfied.

The producer Trevor Horn, who's worked with her on several projects including an upcoming track from the Pet Shop Boys, applauds such obsessiveness. "She's crazy, she's relentless, she never stops working," he notes, "But she always finishes her songs off properly. Most people these days just write round an eight-bar loop; and as a producer, you have to work much harder on a song like that, one that's not properly written. With Diane, she's done all the work and you appreciate it."

It's not only Warren's work that's legendary, though, it's her workspace. When she ushers me into the dark room most people call "The Cave" and gently asks that I touch nothing, it's easy to see how it earned such a nickname. While the rest of her offices are clean and corporate, this room is a mess, every flat surface stacked with hundreds of cassettes; the old white piano she works from peaks out from under the piles, while the windows are hidden behind a pair of tatty curtains. This mess is down to Warren's superstitious nature; she's changed nothing since first writing hits here in the mid-1980s, and rarely allows visitors inside ("I refuse to go into that hole," laughs Simon Cowell).

Unlike many songwriters, Warren prefers to work alone. "I love to write by myself, so I can follow my own heart," she explains, sounding like one of her songs. But she also prefers not to collaborate with performers for more pragmatic reasons: artists often reject songs after they're finished, leaving Warren with a song that no one else will want to record simply because of its byline, which implies low-grade leftovers.

Though her shy oddness makes it easy to underestimate Warren, this business savvy is one of the many examples of how smart and worldly she is. She talks about sync fees and synergy between songs and movies (she's Hollywood's go-to girl for sweeping love themes such as "There You'll Be" from Pearl Harbour and "Because You Loved Me" from Up Close and Personal) and dismisses writing for Celine Dion again, not only for lack of an artistic challenge but because, as she notes, the newly Vegas-based showgirl sells fewer records than she used to.

Certainly, Warren's well aware of her power. She's relentlessly commercial, and watches chart placements closely; she'll never waste a terrific song on an artist or record label that won't push to promote it. She's more interested in sales than praise, and dismisses the criticism that her music is "too commercial".

"It's the funniest thing," she says. "You'll have these great bands - and I'm a fan of these bands - but then they'll say, 'We don't want to sell out and have hits,'" she chuckles. "I think, 'OK, why are you signing a record contract? Why are you putting a record out? Just make CDs for your friends.' I don't understand it. I mean, everybody wants a hit record..."

For all her business skills and ability at hitmaking from heartbreak, Warren is famously single; she calls herself a "100 per cent loner". She says she's never been in love, and that the deep emotions tapped for her tunes come from other places. "I've had my heart broken a lot - you can have your heart broken if a song isn't a hit. I know about heartbreak," she explains.

Toni Braxton, who had a number-two hit with "Un-break My Heart" in 1996 puts it this way: "Diane knows how to take a common human experience and reshape it into a musical message that speaks right to the heart." Steve Tyler, whose group Aerosmith had a top-five hit with Warren's "I Don't Want To Miss a Thing" in 1998, goes even further. He calls Warren "a catcher of dreams that come in rainbows of colours and endless emotions that stick in the hearts of those that hear them".

One of Warren's most personal songs, "Because You Loved Me", wasn't a romantic song, but rather a tribute to her late father; while the longing of "How Do I Live" came from imagining what it would feel like to never be able to write music again. Warren's an admitted workaholic, though she says she finds songwriting relaxing; holidays are rare. "I don't travel a whole lot. I don't like to fly."

She may pass on the chance of a boyfriend or a weekend in the Bahamas, but Warren's passionate about animals; it's said that her closest friend is Buttwings, who often sits on her shoulder. "Parrots are kind of assholes, and I like that. Even Buttwings is kind of a dick - sometimes he'll act like he's cool, then he'll bite you for no reason," she smiles.

Her love of Buttwings's bad behaviour highlights how, for all the tenderness in her songs, Warren has a refreshingly dark and cynical sense of humour. "She's almost like a Jekyll and Hyde," Cowell says. "When you talk to her socially, some of the stuff she comes out with... she's very coarse. I couldn't care less - I love it. But then you hear the lyrics in her songs that are so poignant and beautiful. I often wonder, 'How can she write lyrics like that when she's so coarse when she's out with me?' And the way she writes so deeply about love yet she likes being single..."

Perhaps it's not just talent but fear that makes Warren such a success - she believes it's the only thing she can do. "Otherwise, I'd be a bag lady," she says quietly. "I'd definitely be one of those people you see on the streets. I don't know how to do anything else, I truly don't. I am truly useless at everything else, except this. It's the one thing I'm not useless at."