Gordon Brown will never again have such an opportunity to determine the direction and image of his Government. Prime ministers have fewer levers of power than often thought and picking a Cabinet is one of the few ways they can make their mark, for better, or often worse.

No wonder they spend so much time agonising about reshuffles. Margaret Thatcher devotes four pages of her memoirs to the formation of her Government in 1979 and “the real constraints under which the choices take place”. Unlike presidential systems, ministers must be members of the Commons or Lords. Even Mr Brown’s attempt to reach out to “include all the talents” will only be of advisers, or at minister of state or junior level, with the creation of the odd new peer, as with Mark Malloch Brown as Minister for Africa, Asia and the UN.

But, in general, the parliamentary, and especially Commons, closed shop applies. So we will see none of the “adventurous” big-tent choices made by Nicolas Sarkozy, such as naming Bernard Laporte, the French rugby coach, as Sports and Youth Minister after the World Cup in October.

In his magisterial study, The Prime Minister, Peter Hennessy records Attlee’s personally typed permutations before his September 1947 reshuffle, “his careful concern for age, trades-union background and regional balance”. The National Archives contains a fascinating note from Tim Bligh, Harold Macmillan’s principal private secretary, about the options for “a possible reconstruction of the Government” before “the night of the long knives” in July 1962 when a third of the Cabinet went. Bligh drew up a list of the over55s, with some exceptions to be retained, and said the Chancellorship needs “the right sort of man, young, tough, imaginative, politically strong and publicly articulate”.

The July 1962 changes did not do Macmillan any good. They were seen as a sign of panic, prompting Jeremy Thorpe’s memorable putdown: “Greater love hath no man than this, than that he lay down his friends for his life.” The Brown reshuffle is even larger, with nearly half the old Blair Cabinet going, but will not have such negative consequences. Most of the departures were voluntary, and/or close associates of Tony Blair, who are unlikely to cause any trouble.

There has been a recognition, on all sides, that this week should mark a change of eras. Mr Brown has therefore had greater freedom than many of his predecessors in living up to his aim of projecting an image of change and freshness. The average age of the Cabinet has fallen by five years and the promotions of David Miliband and Jacqui Smith personify the arrival of a new generation, even if the public will take little notice of the other new names.

In political terms, a line has been drawn under the Iraq war, with the return to the Government, and promotion to Cabinet, of John Denham, who resigned in March 2003. Similarly, the new Lord Malloch-Brown has been a strong critic of US policy. These changes are not intended to mark a shift in the British approach, and do not imply a sudden withdrawal of troops. But they will permit a change in language and tone, a greater willingness to admit past mistakes.

The new team balances the promotion of Mr Brown’s close allies such as Ed Balls and Ed Miliband, and the return of Nick Brown, an ex-Chief Whip, in the lesser role of Deputy Chief Whip, as well as the inclusion of proteges of Mr Blair such as James Purnell and Andy Burnham. The old Blair/Brown distinctions are, anyway, fast disappearing, and to talk of a tilt to the left is misleading. Reformers will hold most of the key public service posts, such as the Home Office, Health, and the two new education departments. The main question is over Peter Hain at Work and Pensions since, for all his political energy, he is not seen as a details man.

Mr Brown also announced far-reaching changes in the structure of Whitehall. He has created two education departments: one up to age 19 including family issues, and one post19 including innovation and scientific research. This is a puzzling move at a time when the Government is trying to increase links between higher/further education and secondary schools. Trade and Industry has been transformed into the Department for Business, Enterprise and Regulatory Reform, the sixth such change of function or name in the past 40 years. Enterprise has appeared and disappeared from the title at regular intervals.

Persuasive-sounding reasons are always advanced. But the record is less impressive. There are always big costs from such upheavals, not just financial but as civil servants move around. And the benefits are much less apparent. Few believe that the performance of schools has been improved by the three changes of identity since 1994. Harold Wilson created five new departments in October 1964, three of which had disappeared before he lost office in 1970.

Structure is less important than who is in charge. Baroness Thatcher always ensured that ideological allies, provided they were also energetic ministers, ran the main economic departments. Those that were not up to it were dropped or pushed sideways in the 1981 and 1983 reshuffles.

Mr Brown hopes his new team will remain until the next general election, with only minor variations. However powerful the centre of government– and the Brown team in Downing Street and the Cabinet Office is strong– implementation still depends on the ability and drive of heads of department. The new Cabinet team is longer on promise than achievement. What matters for voters in the long term is not who’s in and who’s out, but performance and results.

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