Meanwhile a small boy - name of Ben, the child of Sue Nye, Brown's political secretary - solemnly munches his way through his third birthday tea between the grown-ups. The irresistible effect of the picture, which was on most front pages yesterday morning, was of a family snapshot. Here they are, Gordon, Sarah and Ben.
No problem. Except, of course, that one problem: the whole world, bar a small clutch of forgotten hunter-gatherers in Wales, knows perfectly that Gordon Brown is single. He has been one of the most bachelorish bachelors on the planet, tousled, rumpled, work-driven, reticent. If policy-obsessives can be laddish, then he is even mildly laddish. Wonderful stories are told of his former Edinburgh flat, where dust covered every surface, there was nothing in the fridge and his idea of dressing down for the weekend was to swap a red tie for a blue tie.
Certainly, he has not yet announced whether, (never mind when) he will marry Ms Macaulay. He seems in no hurry. Why should he be? But given all this well-known background, the family snapshot stunt came across as both poignant and impertinent. Here, after all, are real people with real emotional lives, posing for a pre-Budget photo-opportunity to celebrate the state of family - almost as if they were saying: "We aren't married with a child. But, you never know, we might be; we are those sort of people. We would, at any rate, like you to picture us that way.''
But - please - why? Is the world still, after the sexual revolution, and women's liberation (postponed) and the arrival of gay ministers, supposed to be so determined that leaders be married that endless nudge, nudge stories and stagy photo-ops are thought necessary to enhance Budget day?
The circumstances are unusual, admittedly. The Chancellor was the object of a nasty whispering campaign about his sexuality. Rebutting that was the job of his press officer Charlie Whelan. With characteristic gusto, Mr Whelan then seemed to make his dignified and reserved boss seem like some frantic serial shagger, the living embodiment of erogenous growth theory. Next came endless stories about Ms Macaulay and the imminence of her nuptials, spoilt only by Mr Brown's habit of sloping off to watch football and his frank pleas for a little space and time. She, meanwhile, is in a hellish position, and handling it with extraordinary aplomb.
Damned if they do, damned if they don't? No pictures of Sarah Macaulay would presumably lead to "Sarah snubbed on Gordon's big day''. On the other hand, why should they let it all hang out? They aren't trying to promote films or sell records.
So they opted for being vague. Brown says that "marriage is important'' and that "I'm sure I'll do it one day'', making the whole business sound like passing a driving test. Then came that strange remark about it being wrong to force anyone into marriage.
It's strange only because, these days, almost no one in this country is. One has in one's mind's eye a picture of burly Kirk elders dragging a frantically resisting Chancellor into a soot-grimed church on a hill in Fife. And yet, as a country, we marry less and we marry for fewer years. If we were as family-based as we used to be, there would be no need for four million extra homes. Singles culture is celebrated by television and flattered by advertisers who know how to spot personal disposable income when they see it.
Yet Brown's image-makers seem to disagree. It is partly that a man who will today be changing the incomes of so many families, possibly through mortgage tax relief and child benefit, has to seem as if he knows something about contemporary family budgets, and, therefore, about contemporary family life. He doesn't want to be seen as the dour theoretician of Downing Street. He needs to be homely.
Mr Brown's neighbour in Downing Street shows how a politician's family can be used positively by the media. The Blairs have become almost a kind of alternative Royal Family for the mid-market press and mid-market television. We know when they change their car. We know which football teams their kids support. The Daily Mail, in relatively benign mood, follows the clothes and hairstyles of Cherie with almost as much fascination as it once followed Diana.
These may be ominous echoes, but the press requires recognisable people to at as mannequins for lifestyle journalism. The spouses and children of the famous become handy pegs for articles about education, clothing, holidays, health and furnishing. In return, it is assumed, the famous benefit from the reassuring glow of normality that such pieces confer. My point is merely that for politicians, or any other public figures, to respond to this in a substantial way is deadly. To place your emotional life at the service of good PR is coming close to selling your very identity.
I don't believe for a moment that most voters care whether Gordon Brown is going to marry Sarah Macaulay or not. They will read stories about that if such stories are there, and look with some interest at the pictures offered up. That's natural curiosity. Some of us, maybe, associate family with stability, long-termism, responsibility, and therefore, trust politicians with children more than politicians without children.
But integrity matters much more. We respond best to politicians who seem to be themselves. John Prescott, Clare Short and Ken Livingstone are all more popular than many drably personable clones because "you know who they are''. In this case, Brown really has been more interested in policy, power and economics than in personal relationships. Other people spend their energy chasing folk round the filing cabinets; he's mostly been too busy reading whatever's in the filing cabinets. Given his job, that is surely reassuring. To his real friends, it is a lovable trait. And only in a strangely sex-obsessed and voyeuristic age would this be thought so worthy of disapproving comment.
At any rate, his married state isn't going to affect a single vote or change a single mind when today's Budget is unveiled. People in families will applaud Brown if he helps their weekly incomes and curse him if he taketh away. No photo-calls with children or coy hints about forthcoming marriage will sway those reactions. We are not so babyish that we can be bought off by the thought of the Chancellor's bride - not when he affects our pockets. Ministers should not think that suiting the press means pleasing the voters.
So maybe the Chancellor will get married and be happy, and that will be fine. I have a vague but unprovable idea that happy politicians are better for us than unhappy ones. But for me the most reassuringly human aspect of Brown's pre-Budget photo-call was his seething unease about the whole damn business.