“My project will be complete,” Tony Blair once famously said, “when the Labour Party learns to love Peter Mandelson.” Mr Blair did not make as much progress with this endeavour as he might have hoped. Now, it seems, the Labour Party and the British people are to get another chance.

Reshuffles are usually judged on their shock value, and this one scored highly. It was a media coup, and makes the Brown administration look more exciting at breakfast this morning than it was at breakfast yesterday morning. But this is the wrong way to assess the changes. By next week little will be left of the advantage gained by surprise. The reshuffle will be judged on whether it leaves the country better governed. And the picture is likely to be mixed.

The most positive thing to emerge is a better structure for Government. The appointment of a dedicated defence secretary, unencumbered by other positions, is overdue and welcome. The new post for Ed Miliband sensibly brings energy policy and policy to prevent climate change under the same Cabinet minister. In turn this provides greater clarity for those managing policy towards business and farming, which now have ministries of their own.

A second advantage to the changes is the decision to redeploy Mr Brown's adviser Damian McBride, the man who has been briefing the press. To be effective, the Prime Minister's spokesman must retain the trust of his senior colleagues. This had been lost. He had a hard job at a difficult time, but occasionally Mr McBride took the mythology of a hard-bitten, tough-talking attack dog just a mite too seriously.

In the end, however, the success of yesterday's changes will depend on whether Peter Mandelson's return makes a positive impact on the Government. And this is deeply questionable.

Mr Mandelson is formidable. He helped to make the Labour Party not simply electable, but also deserving of election. Even his titanic feud with Gordon Brown, distracting as it was, and his two resignations, avoidable though they were, do not cancel out the good he did.

He returns, however, to a very different political climate from the one he enjoyed in his heyday. His ability to get his way with the press and his party depended on new Labour's domination of the political scene. He and his close allies were the new stars, with all the fascination and deference that such political celebrity brings. Now he is a scarred veteran rejoining an embattled Government. The new stars of politics are to be found elsewhere and the benefit of the doubt he once was given is being given to others.

So Mr Mandelson faces a difficult task and will have to show great self-discipline - not the quality for which he is best known. It is easy to see his attraction to money and glitz landing him in trouble again. And his own MPs, leery in any case of a Cabinet minister in the Lords, will be a tough crowd to please.

This latest twist in his turbulent relationship with Gordon Brown is reminiscent of the second marriage of Richard Burton and Elizabeth Taylor - a break in the clouds after a stormy partnership. But that lasted only ten months before descending once more into acrimony.

Mr Brown has decided to risk it. It is not hard to see why. With Mr Mandelson in his Government he is, in the short run, much less likely to be evicted from office by a Blairite ministerial coup. But his ultimate fate will depend on his making a far deeper decision. What sort of Prime Minister does he wish to be?

Is the return of Mr Mandelson a decisive shift to become a determinedly reformist Government? Or just a piece of party and media management? If it is the latter it is more likely to fail than to succeed.