NECESSITY, ACCORDING to Mark Twain, is the mother of taking chances. That better puts into perspective Gordon Brown's "gamble" of the last few days when he chose to bring back Peter Mandelson from Brussels, give him a key role in his Cabinet, and bury the hatchet that has divided the pair since their fall-out in the early days of New Labour.

Ed Balls, the schools secretary and Brown's closest ally, admitted Mandelson's appointment was a risk that had been carefully calculated. So let us look at the political arithmetic of this risk: The first option is always to do nothing. Brown is a practitioner of non-action. But such is the state of his government that doing nothing is no longer an option. Labour face a landslide defeat to the Tories at the next general election, and no-one in the Cabinet seemed to know how to stop this happening.

Replace Brown? No-one, especially not David Miliband, has the bottle to tell Brown he might yet destroy the thing he helped create, New Labour, and most of the Labour movement if the worst polls become reality. So change there had to be. Short of asking Blair to come back, Mandelson was the next best thing.

For someone who said he once gave Mandelson 20p to phone all his friends, and who accepts party unity remains a big challenge, offering Mandelson a Cabinet job indicates the scale of Brown's problems. Mandelson is a divisive figure, more Machiavelli than Mandela. But to survive until the next general election requires the ultimate sacrifice, and where Balls sees risk, others will see capitulation: an end to Brown's version of New Labour and a return to the Blairite project which targets the centre and middle ground, and has widened the gap between Britain's rich and the poor over the past decade.

Mandelson at the centre of Brown's government isn't a risk, but a calculated assumption that what worked in the 1990s when New Labour defeated John Major can work again. Brown's house is crumbling, so he's brought in the old builders - Mandelson and Alastair Campbell - with Blair a phone call away.

The global financial crisis, in which novices can't be given responsibility, has given Brown a narrative in which to explain his government's problems. Yet such crises were meant to have been banished. Boom and bust, the old cycle, Brown said, had gone. But it's back and his new calculation is whether or not his old enemy, the inventor of spin, can be resurrected to help him defy the polls.

No place for novices? Where does that leave Jack McConnell and his new job in Africa? If it is all about experience and tested expertise, how does a spell in Holyrood equip you for international conflict resolution, working alongside the UN? It doesn't, and Number 10's move is another cynical calculation to avoid a by-election and damage a Brown relaunch.

The appointment of Jim Murphy as Scottish secretary has little to do with what's best for Scotland. Brown needs a believable narrative for the Glenrothes by-election, and has appointed Murphy to a part-time job from his busy post as Europe minister. Murphy comes close to saying he knows his new job is more presentation than policy. This year it has been Paris, Berlin, Madrid and Rome. This week, he says, it'll be Arbroath.

If Mandelson is back on form, Murphy's phone will be ringing this morning warning him to be more careful in future. Mandelson is back. The only real questions are for how long, and will it be enough to save Brown?