Next month, Christine Gilbert will become the sixth chief inspector in England in the 14 years since Ofsted was formed. In the previous 102 years, there were just 12 senior chief inspectors . She will need to draw on all her experience as a secondary head, chief education officer and local authority chief executive to do a job that has grown substantially in the last two years.

Ever since the first Her Majesty’s Inspectors were appointed in the 1840s the position of chief inspector has required a delicate balance. For most of that time their independence from the government has been emphasised by being Her Majesty’s, not the Education Secretary’s inspectors, with their role in schools being to evaluate and comment, not to instruct.

This was made clear with the first “Instructions to inspectors” in 1840, which stated: “You should bear in mind that inspection is not intended as a means of exercising control, but of affording assistance; that it is not to be regarded as operating for the restraint of local efforts, but for their encouragement, the inspector having no power to interfere, and not being instructed to offer any advice or information except where it is invited”

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While the focus in the Ofsted years has been on the inspection of individual schools, it is an equally important role of the chief inspector to comment on the effectiveness of our highly centralised national system.

That means sometimes biting the hand that appoints you.

What sets the inspectorate apart from other educational commentators and enables them to carry out this part of the job is, or should be, the wide evidence that it acquires in the course of its daily work. In Ofsted it has not always been so.

The TES described Chris Woodhead’s first annual lecture in 1995 as “an entirely fact-free zone. Second-hand eloquence rather than first-hand evidence”. Neither governments nor teachers are well served by a chief inspector with a personal agenda. Both are helped by a chief inspector whose words are based on the comprehensive data built up by Ofsted from several thousand inspections each year.

Under Ofsted, the inspectorate has become too closely associated with government policy, but the chief inspector should not be the secretary of state’s enforcer. The best chief inspectors have always stood between the profession and the government, independent of both and commenting only on the basis of evidence on the successes and failures of teachers, heads and government policy.

There is no shortage of evidence. In recent years schools have become data-rich, developing increasingly rigorous systems of self-evaluation, incorporating the views of parents and students. So it is right that inspection should now be seen as part of the new relationship with schools, in which the old Ofsted quality control model has been ditched in favour of quality assurance, with external inspection tied much more closely to internal school self-review. The next stage is surely for inspections to be focused more clearly on just two questions: “How is the schoolcollege doing?” and “Does it have the capacity to improve?” That will help the system to become more of a validation of school or college self-evaluation.

Only then will it begin to move from a low-trust to a high-trust model.

Inspectors must use data more intelligently. The link to self-evaluation must be firmer. More heads and experienced teachers should be on inspection teams. Proportionate inspection - a sensible use of resources at the top end - should mean more support, not more inspection, for schools judged “satisfactory”.

The 2005 inspection framework represents more intelligent accountability than the 2003 model, but it is still too often carried out in an atmosphere of fear. An experienced and successful headteacher wrote to me recently: “I love teaching and headship, but I am retiring five years early after the most awful inspection last year. The lead inspector was a total bully. I vowed I would never face another Ofsted inspection.

“You don’t know what they are doing with data and how you will come out - I can’t trust that level of uncertainty. Nothing I’ve seen in the last year has convinced me that it will be any better in the future.”

The profession can ill afford to lose such people.

By contrast, Martin Roseveare, the chief inspector in the 1950s, wrote to new inspectors: “I am sure that you will always remember that you are His Majesty’s Inspector of Schools, that consideration for others, and courtesy will be expected of you at all times.”

All of that is needed in the high-trust model that the new chief inspector must develop.

Dr John Dunford is general secretary of the Association of School and College Leaders and is the author of Her Majesty’s Inspectorate of Schools since 1944: Standard Bearers or Turbulent Priests? (Woburn Press, 1998)