It was a warm-up no one expected – even the man being introduced appeared all delighted surprise. Just before Gordon Brown stepped up to the podium at last week's Labour Party conference to deliver the most crucial speech of his political career, his wife Sarah emerged from the shadows. Her tribute was brief, but with each word emphasising her respect and admiration. "I am so proud that every day I see him motivated to work for the best interests of people all around the country," she said before introducing "my husband, the leader of your party, your prime minister… Gordon Brown."

Never was a man normally associated with dull predictability so much in need of a charisma boost. And the fact it came from the most unpredictable source – his intensely private wife – so delighted the party faithful that Sarah Brown was given a two-minute standing ovation. The couple kissed before the prime minister opened his speech in equally uncharacteristically personal mode.

No one was surprised at his declaration that he "didn't come into politics to become a celebrity, or thinking I'd always be popular." But it was the admission of it that emphasised his honesty in a party notorious for spin. "I'm not going to try to be something I'm not," he added. The words could have been Sarah Brown's too.

Even before she was reluctantly cast in the spotlight as the first lady of Downing Street, when the 'Iron Chancellor' became a worryingly leaden prime minister in 2007, her character, never mind her wardrobe, has been scrutinised and picked over. So far, she retains a solidity and apparent lack of vanity, described as a woman who simply wants her high-profile husband to do well, the perfect "political wife". But she is no Tammy Wynette, simply standing by her Scots Presbyterian man.

During her teenage years, she might even have been a bit of Peaches Geldof: at Camden High School for Girls, she is remembered as a ringleader of 'the Trendies', a clique known to spray themselves with gold body paint before heading to a party. She was also recalled as "posh" and "poised". Her husband, 12 years her senior, once claimed, "My wife is from middle England." A version of that old-fashioned poshness remains in her prioritising of motherhood over career.

But although visitors to No 10 might stub their toe on a random piece of Lego left lying around by the couple's two small boys, the notion of her in a tartan pinny making shortbread or knitting tam o'shanters would be way off the mark. She may not have a highly paid, high-profile profession like her No 10 predecessor Cherie Blair, but Sarah Brown's commitment to a number of charities, ranging from domestic violence organisations to cancer care centres, takes up any spare moment.

It was her founding of Piggy Bank Kids in 2002 that added a personal dimension to her strong ethical drive. Jennifer Jane, the couple's first child, died in her parents' arms just 10 days after her premature birth. Gordon Brown's normally stoic guard was briefly dropped, evoking great sympathy. Heartbroken Sarah turned the tragedy into a cause, initially to research complications in pregnancy, but later widening it to help disadvantaged children. The birth of the couple's son John in 2003 brought much deserved joy.

When the Sun revealed their second child Fraser (born 2006) was diagnosed with cystic fibrosis, the normally private Browns appeared somewhat relieved it had become public knowledge, and are said to be positive and upbeat about his future.

Before she entered No 10, Sarah Brown declared to friends she wanted to live "under the radar." Brown numbers among her friends the writer JK Rowling and columnist Mariella Frostrup. "She's just not vain," Frostrup remarked. In a conversation the day before a previous Labour Party conference when Gordon Brown was chancellor, Frostrup recalls her pal sighing wearily, "I really must get out and get a suit or something." The label tends to be dependable Jaeger, for which read "timeless and understated."

Friends say the Browns live fairly frugally, and so any clothes purchased are meant to last beyond the vagaries of fashion. When the couple married in the summer of 2000, it was a low-key affair at Brown's Queensferry home. Although the soon to be Mrs Brown wore a silk two-piece by Louise Kennedy, it was bought quietly off the peg rather than with the usual fanfare of fittings in a designer salon. "I had no idea Sarah Macaulay had chosen this outfit," said a flattered Kennedy afterwards, adding that the style of the jacket and shift dress was "classic Grace Kelly."

That Sarah Brown detests "the triviality" of newspaper stories such as her buying pastel ties to soften her husband's solemn image, or giving him an iPod for Christmas, may sound surprising given her former career in PR. But the psychology graduate wasn't interested in launching lipsticks or puckering up to fashion editors with freebies – this was to be 'PR with integrity', raising the profiles of charities, trade unions and, presciently, the Labour party.

She had her first serious conversation with her husband-to-be on a flight to Scotland in l994. The romance only became official in 1997, when, bowing under pressure from New Labour spin doctors concerned about tabloid speculation on his bachelor status, or that he might be gay, Brown reluctantly agreed to a staged photo opportunity with Sarah in a London restaurant.

His irritation at this publicising of his private life was so ill-concealed the picture had to be re-shot showing him more suitably besotted with his new partner. She has long since developed the skill to head off the things that annoy him.

"Gordon always used to lose his mobile, something was always irritating him," David Blunkett has said. "She keeps him calm."

The beleaguered prime minister
last week claimed the Labour Party would prove a "rock of stability." For him, his 'rock' is his steady, tranquil wife, reluctantly but dutifully summon-ing up those PR skills abandoned when she first became Mrs Brown, but which may yet play a role in his political survival in the dark times ahead.

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Born Sarah Macauley, October 1963, Buckinghamshire.

Career Co-founder of PR company Hobsbawm Macauley; charity worker

Personal life Married Gordon Brown, August 2000; two boys, John (5) and Fraser (3)

In the news The British PM's resolutely private wife gave a surprise speech to introduce her husband at the Labour Party conference