In ten days her husbandwill become prime minister of our nearest neighbours. So who, exactly, is Sarah Brown?

THE wait is almost over. The keys are almost in her hand. So let us put ourselves in the mind of the satirical artist Alison Jackson and imagine Sarah Brown standing with her back against the door of No 10, Downing Street, punching the air. Next, she goes upstairs to the White Drawing Room, traditionally commandeered by the PM's wife. She surveys the room. Hers, all hers. Then her eye alights on something twinkling in the window. A quartz crystal charm, forgotten by Cherie Blair.

She swoops, she snatches, and with a deft overarm, sends it flying into the bin. . .

Who knows how Sarah Brown will make Number 10 her own? It's a difficult move to make. The morning Mary Wilson arrived, she was so nervous she was physically sick. Clarissa Eden discovered a parliamentary private secretary sitting in her bedroom in the early hours of the morning. And Norma Major wanted to re-decorate the kitchen but, afraid of public censure over the cost, had it done gradually, one unit at a time. Life as the prime minister's wife is strange indeed.

"Nothing, " according to Cherie Blair, "prepares you for the reality of what awaits you behind that famous front door."

But if anyone is prepared for the challenge, it is Sarah Brown. She has certainly had ample time to think about what colour she wants the curtains.

Or rather, which charities she is going to support.

Mrs Brown is not interested in trivialities of decor; at Number 11 she purged with puritan zeal the Gainsboroughs and hung up portraits of trades unionists instead. She will be a seriousminded First Lady, one feels. The job will mean a lot more to her than wearing red accessories.

Sarah Brown has never given an interview.

Discretion? Fear of putting her foot in it? Both, probably . . . as well as basic modesty. She is an unassuming presence around Whitehall, often spotted weighed down with shopping bags and baby buggies. But behind her muted public persona there is an interesting woman: clever, well-connected . . .

fun, even, judging by the kiss'n'tell story about her heady youth written by a rock musician called Babel. But not too fun . . . Babel's revelations weren't so very shocking. Whitehall could breathe easy again once they knew the worst of it: she once sewed him a pair of leather trousers.

Quietly supportive When Sarah Macaulay met Gordon Brown, she was already respected in her own right for creating "integrity PR": noble causes, honestly presented. This was "a very clever idea", according to social commentator Peter York, and Sarah and Julia Hobsbawm, her partner, were good at it, with a client list including the V&A and the New Statesman. But when the business clashed with her status as Mrs Brown, she resigned and took up motherhood and charitable concerns instead . . . activities considered more suitable for a First Lady-inwaiting. You could see this as the traditional female career sacrifice, made on the altar of her husband's job. But in fact it was probably more like the exchange of one ambition for another. She threw herself into Project Brown, taking on his name and his leadership campaign.

The way she manages Brown has been much admired. One anecdote sums it up best. At a dinner party, he was sitting at the table in splendidly silent isolation. His wife sent him a note saying "Talk to the women on either side of you." (He returned the note with the scrawl "I have.") "Gordon always used to lose his mobile phone, something that was always irritating him, " says David Blunkett, the former Home Secretary.

"She keeps him calm."

The novelist Kathy Lette, who has known Sarah Brown for 18 years, says: " I can't think of a more quietly supportive person than Sarah. She's amazingly anchored. She has a low centre of gravity. It would take a lot to shove her off balance."

Spouses of politicians can exert crucial influence: supporting, prompting, humanising. Sarah has given Gordon another dimension beyond his intellectualism. He once said he read economic treatises in the bath; now he is more likely to talk about bathing his sons.

Sarah and Gordon are a match made in Scotland. She too comes from a line of rectitudinous Scots, with a family history of public duty.

"Her grandfather was a knighted Scots missionary, " says Peter York. "Can you think of a more appropriate antecedent?"

She was born in 1963; her father Iain was a publisher and her mother Pauline a teacher.

Sarah's early years were spent in Fife, before the family spent a few years in Tanzania. Gordon has quoted her childhood stories while speechifying on official visits there. Sarah refers to a "tough" period when she was seven and her parents divorced, leaving her mother bringing up Sarah and her two brothers alone. Sarah now supports the National Council for One Parent Families . . . with her friend JK Rowling, who recently wrote a new short story for the fundraising anthology that Sarah co-edited (Magic, Bloomsbury, 10.35). Their relationship seems to go beyond the usual acquaintance of the famous with the powerful: Rowling visited Sarah and Gordon at the Royal Infirmary in Edinburgh the day after their son John was born.

The family, always middle class, became more so when Sarah's mother remarried an eminent professor of tropical medicine. Sarah could have gone to private school but chose the left-leaning north London literati's school of choice . . . Camden Girls. How at home Sarah seems amongst the ranks of that institution's bien pensant, highachieving alumni. Her contemporaries from school are a loyal lot, reluctant to comment about her other than to describe her as a "stunning, willowy, hippyish 17-year-old who would often walk about with bare feet." In her teens, Sarah met many who make up her present milieu: Julia Hobsbawm, daughter of Eric Hobsbawm, the Marxist historian; Arabella Weir, the comedian and writer; Toby Young, the man who lost friends, alienated people and wrote it up with great success . . . as well as one Helen Scott Lidgett, who taught her art.

Educated, connected, motivated This woman has been touted as a possible heir to Carole Caplin. Dark and well-presented (she used to run a vintage clothes stall at Camden market), Scott Lidgett has been a constant presence in Sarah's life, making the Miss Brodie transition from teacher to friend. When Macaulay and Hobsbawm set up shop, she was invited on to the staff;

when that company disbanded, she followed Sarah over to Brunswick PR. Scott Lidgett is also a trustee of Sarah's charity, PiggyBankKids.

But the idea that she might become a Svengali figure is denied by friends. "She's too confident to want someone dressing her. She instinctively knows what not to wear, " says one; Charlie Whelan, Gordon Brown's one-time press secretary, a tough, bluff personality who resigned after being accused of leaking the Mandelson loan scandal, agrees. "Sarah won't need to depend on anyone."

(Although he has already dropped the hint in his column in PR Week that if she is looking for an advisor, she could do worse than choose a male former spin doctor. . . ) At Bristol, where she took a 2:1 in psychology, she reportedly fell in with a "posh, even aristocratic set". There were parties, some of which she attended painted in gold body paint, others not.

The rumour that she dated Alex Cameron, David's brother, is, disappointingly, unfounded.

After university, Sarah had a relationship with Falklands veteran Ken Lukowiak, who lived with her after his dismissal from the paratroop regiment for smuggling cannabis. He described his time with her as "one of the best summers of my life, like being in a rock'n'roll movie". A friend explains diplomatically: "She's always managed to combine being high-minded with being friends with people who are fun and off the rails. She knows how to manage them."

Next, she went into career drive, working ferociously hard. Sarah was a yuppie in the true sense of the term . . . educated, connected, motivated. Her first job was with brand consultancy Wolff Olins, specialising in Labour gigs, such as the Manufacturing Matters Conference (oh, the glamour) and party donor events . . . where she came into Gordon Brown's ken.

'A lovely lass' According to Tom Bower, author of an extremely unauthorised biography of Brown, Sarah set her cap at Gordon, keeping a file of cuttings on him for 18 months. This behaviour seems less Machiavellian if you consider she was a conscientious PR worker at the time. Their first conversation, on a plane up to Edinburgh, was about Fife. Where else?

Sarah, crucially, had the approval of "Gang Gordon, " his political secretary, the long-serving Brown loyalist Sue Nye, and Charlie Whelan.

Her early dates with Gordon included drinks in the private members' bar Soho House, but their relationship remained discreet for years. Too discreet, perhaps. In 1996 on Desert Island Discs, Sue Lawley sprang a surprise on Gordon. "People want to know whether you're gay . . ."

But Brown was unhurried by public pressure.

His girlfriends before Sarah included newsreader Sheena Macdonald and the exotic Princess Margarita of Romania. But "he never committed, " according to a friend. " Sarah got him to commit, finally. She didn't issue ultimatums, she let him come to her in his own time." Still, it was a long and tortuous courtship. Even John Prescott tried to play cupid. At the 1998 party conference, he got up on stage and said "Gordon, forget prudence and name a date for Sarah. She's a lovely lass." Sarah must have turned a truly socialist colour.

Passionate and Committed It was just before Brown's 1998 budget that they finally went public. Charlie Whelan gave the tipoff to a tabloid photographer that Brown and Macaulay would be dining together at Vasco and Piero's restaurant in Soho. They look as if they are discussing, over a cosy post-prandial cappuccino, post-endogenous growth theory. Apparently, the photographs had to be shot twice in order for them to look sufficiently natural. One wonders what the first attempt looked like.

When they married in Fife in 2000, word got out at the last minute and their faltering public kiss was caught by the assembled media. Guests drank Sainsbury's own-brand champagne, inspiring a local paper's cruel headline " A fine day for a tight wedding". At a much grander wedding party in London, Brown gave a witty speech based on the euro debate. He and Sarah had agreed "in principle" to marry, but he had to pass five tests first, including "convergence" and "longterm sustainability" .

Sarah had always been more behind-the-scenes at her company, Hobsbawm Macaulay Communications, once commenting, "Julia goes out to lunch so I don't have to" . . . but complaints in the press about Sophie Wessex using her marital connections to promote her PR company led to similar claims about Sarah. Her clients, often institutions funded through the Arts Council or the Lottery or both, were "employing Mrs Brown in the hope of receiving even more public money, " fumed Bruce Anderson in the Spectator in 2001.

Sarah was then "ringfenced" by Gang Gordon. She left HMC; shortly afterwards, the business folded. The settlement between the co-founders was said to be acrimonious. Julia Hobsbawm has excised all mention of the name Macaulay from her CV on the website of her new PR consultancy.

Sarah had also retired from her business for a happy reason: her first pregnancy. The death of the baby, Jennifer, 10 days after she was born, provoked great national sympathy. When Gordon and Sarah left the hospital looking broken, the public suddenly perceived them as people, rounded humans. In the period after the bereavement, Sarah kept busy, personally answering the 14,000 letters of condolence, and founding PiggyBankKids, which raises funds for children's charities. She is increasingly visible as a patron . . . this month speaking at both the First Women Awards and a Mother at Work event.

"Sarah is passionately committed to her charity work, so often uses a casual gathering at her home as a way to thank her various contributors, " says Lette. "She is immensely thoughtful like that, never leaving anyone out."

At present, Sarah Brown combines these events with a relatively normal life. She takes her children along with her to most events and shops alone.

This life will change when she moves into Number 10 in 10 day's time.

We have grown accustomed to Sarah Brown's face, but we have yet to hear her voice. How, in fact, does she speak? Shrill patrician? Scots? In fact it is quietly Sloaney . . . but whether she will use this voice for interviews is another question. According to Whelan, "She's got a Queen Mother approach . . . rise above it, keep quiet and everyone will think the best of you." Or, in Kate Moss's case, the worst of you.

She is about to undertake a job that made Norma Major occasionally want to " walk into a lift and scream"; a job that made Mary Wilson ask if they couldn't get a dummy, put a nice hat on it, stick a bunch of flowers in its hands and use it instead of her. But Sarah Brown will doubtlessly cope. And she will have learnt at least one thing from Cherie Blair: never answer the doorbell in your nightie.