At last a successor has been found to the Bank of England, in the form of the new regulator of exam standards. Appropriately, the announcement was made by Ed Balls, who was one of the main architects of the May 1997 decision to make the Bank independent and responsible for setting interest rates. That move is universally accepted and imitated. Ever since then, the Brownites have been searching for ways to apply the Bank model.

Such is the suspicion of mainstream politics that the surest way for any politician to gain approval is to announce that he is passing over responsibility for an important function to an independent body. There is a paradoxical desire to hold politicians to account and to find nonpolitical solutions.

The problem has always been to define areas where goals are sufficiently explicit so that a division can be drawn between deciding a policy, and setting a remit, and then implementation. That is possible on monetary policy, where there is both a clear target in the rate of inflation and a single instrument in the form of interest rates. So everyone knows where they stand.

Similar approaches have been adopted with the Food Standards Agency and, most recently, the Office for National Statistics, although only after arguments about how independent the office should be from ministerial intervention. Mr Balls has argued that in each of these cases independence has been crucial to establishing public credibility.

The same factors apply to the Qualifications and Curriculum Authority, which has been under the direct control of Whitehall since 1988 in various forms. This has meant that ministers have been in the firing line over exam results, with charges that GCSE and A-level grades are worth less than previously.

Since ministers cannot convince the public, there is a strong case for strengthening the regulator and making it visibly independent. This involves splitting the QCA’s current dual roles for curriculum setting and standards monitoring. The regulatory role can be put at arm’s length, primarily accountable to Parliament, while decisions on the curriculum are inherently political and have to involve ministers. The decision has been widely welcomed and has been urged for some time by the Tories and the Liberal Democrats.

But the Bank/arm’s length model cannot be applied everywhere. NICE has operated as an expert body, while facing controversy in deciding that drugs will be available to NHS patients. Mr Brown’s allies have talked about an independent board to run the health service, but that is not workable because most of the key decisions on allocating resources have to be taken by ministers.

The limitations have been shown by the Northern Rock affair. The elaborate tripartite arrangements were bound to be superseded when there was a run on the bank. Moral hazard matters. But there is an inherent difference between the position of depositors and shareholders. In these circumstances the Treasury was bound to become involved and offer a guarantee. You can certainly take the partisan politics out of some areas and an independent body can provide reassurance and credibility. But voters still expect their elected leaders to take big decisions where there is no consensus and to deal with crises.

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