On Tuesday, we may get a budget that focuses on women and children, but as the hour of the photocall on the doorstep of No 11 Downing Street looms, Brown is especially conscious of the acute interest in the women in his own life, not to mentionpotential children. I tell him this as he perches on the edge of a chintz sofa by the fireplace, head down, peering through knitted brows at his chewed fingertips.
OVER the years, Brown has had a series of discreet relationships with glamorous career women, and he has now found happiness and stability with 34-year-old PR consultant, Sarah Macaulay. He has agreed to talk about their relationship, but it's clear this isn't going to be easy.
I mention her name and there is a sharp intake of breath. Can it really be so painful to talk about his private life? No one doubts the Chancellor has hit the ground running, as he intended, but surely he can afford to lighten up and allow a glimpse of his private side?
Although he is breaking the habit of a lifetime by agreeing to pose with Sarah at today's birthday party for the three-year-old son of Sue Nye, one of his top aides, Brown fears he will be damned whatever he does, so he has chosen to put up with the image of the curmudgeonly bachelor rather than expose himself to the suspicion that he is defliberately trying to soften public perception by parading a beautiful girlfriend.
Although his relationship with Sarah Macaulay had been known to journalists for some time, the first evidence of the couple together - a photograph taken of them dining at an Italian restaurant in Soho last year - was greeted by some sniping. How convenient, cynics said, that she works in PR. Was the relationship part of an image make-over, an emotional Colour Me Beautiful?
In fact, Brown first met Sarah when she was organising a Labour fundraiser for John Smith in 1994. Soon after this they found themselves sitting next to each other on a flight to Scotland, and it appears that that was that. So what first attracted him to Sarah? "We were talking about all sorts of things," he mutters, still staring at those nails. "Such as the fact that part of her family came from Scotland, and... er... what kind of work she did. Sarah is a very dynamic woman, she does lots of voluntary and community activities, and I think that's very... erm... good."
Attempting to pierce an Iron Chancellor's armour is hard work. As he squirms and avoids my gaze, his suffering is so transparent I feel faintly ashamed. Those closest to the couple assume they will marry eventually, but neither wants to be bullied down the aisle just because Brown is a public figure.
But outside their long working hours - she runs her own PR company with an old school-friend, Julia Hobsbawm - they are almost inseparable. Though she eschews the role of formal hostess at official functions, she is often there to welcome guests at receptions at No 11, and she is a frequent visitor to his home overlooking the Firth of Forth. Even during weekends together, both work part of the time: Sarah because her company has an office in Edinburgh, the Chancellor because he's in his constituency.
In recent months, the builders have put in a new kitchen: "There are some things I ignored for quite a long time," he says. He appreciates the difficulties Sarah would experience as a political spouse. "Staying in two places at once is what makes it really difficult, but Sarah is very dynamic and very energetic. She's been a very good friend to me." He never mentions the word girlfriend. Last summer, they spent a three-week holiday together in Cape Cod where Brown, his brothers and their families go most years. At Christmas, she met his parents in Scotland.
Since he's been seeing Sarah, the Chancellor has undergone a physical transformation. He lost a stone-and-a-half through daily sessions on the treadmill at his Westminster gym and he appears more relaxed. She tries to steer him away from late-night take-aways with his close-knit, strongly masculine Treasury team to more wholesome fare. Her rice pudding (made with semi-skimmed milk) is, he admits coyly, a particular favourite.
Neither seems anxious to alter the terms of their relationship. "Sarah and I are very fond of each other. She is a close friend [he repeats himself], and I value the time we spend together." Before I get a chance to use the M-word, he hurtles towards it, talking fast: "Marriage is important, and I'm sure I'll do it one day. It's not something anyone should be forced into. We're together and we're happy. That's all there is to it."
Brown is profoundly uncomfortable with demands that he make public utterances of affection. "When people vote for a government they've got a right to know what sort of people they're going to get, but there's obviously a dividing line between what's private - what people would want to remain private about their own lives - and what's relevant to the choice of the politician."
Surely a declaration of his love for Sarah would assuage curiosity, rather than feed it? "I don't think people are terribly interested in my private life," he says disingenuously. "They know where I come from, they know what I do in politics, they know my attitudes towards all sorts of things. I've never made any secret of any of these things. I've been quite open. But sometimes people think there are things to say when there are not things to say."
Beforehand, Brown's press secretary, Charlie Whelan, has mentioned - in one of the loudest stage whispers recorded outside the West End - that Brown got "sackloads of Valentine cards" last month. Women are drawn to his dark, moody looks and the timbre of his voice. They delight at the way his determined jaw drops when he calls for prudence. "Prudence is the other woman in Gordon's life," says one aide.
BROWN may be in danger of becoming New Labour's sex symbol. One profile, which described him as "the handsome personification of brooding intelligence", went on to compare him to Darcy, Byron and Liam Gallagher.
But the tag of being "a psychologically flawed" batchelor whose only joy is to bear down on inflation - in the memorable phrase of one of Tony Blair's spin doctors - has also stuck. Which one is bears the closer relationship to the real Gordon Brown?
"Well, the second one is rubbish. I don't know about the first one, that's for you to judge." The idea made him giggle. "No, it's amazing. I'm not married not because I don't want to be married but because it just hasn't happened. People draw all sorts of conclusions if you're a man in your thirties or early forties. I think it's a bit unfair but it's something you're bound to accept."
Yet his image as a workaholic with no personal life has failed to win him much public sympathy. It was also one of the factors, after John Smith's death, which shifted the party's esteem to Blair.
While the Prime Minister is at ease in front of the cameras, Brown can appear two-dimensional. But nothing will tempt him into criticism of his boss. He launches into a de rigueur tribute to the Prime Minister. For his part, Blair recently eulogised his chancellor, praising his "intellectual firepower" and calling him "my Lloyd George" - a particular tribute, since the creator of the welfare state is Brown's hero. Like his hero, he wants to use the budget as an instrument of social change and hopes to go down in history as a great reforming chancellor. Lloyd George, of course, became prime minister.