He also took me for art history, in which I had had a strong interest from the age of 13, and for a tremendously obscure A-level in Roman history and Latin translation - so obscure that Marlborough was the only school in the country that offered it.

At the time it was impossible to fathom how old Peter was. I know now that he must have been in his mid-forties, but there was already a sense of venerability to him. As far as we were concerned, he had been at Marlborough since the beginning of time (though he had in fact joined just as he was finishing an Oxford DPhil).

He had straw-coloured hair, warm features and was completely donnish. In fact, in his broad-ranging intellect and his belief that the life of the mind was the only thing of any importance, he was more donnish than any don I’ve ever met. His style of teaching was certainly more like that of an Oxford lecturer than a “conventional” school master. I can’t remember him ever asking us a single question or encouraging us to participate in class.

It was a method entirely contrary to today’s ethos of gallery education, which is all about getting pupils to engage with and respond to the works of art on the walls, rather than read or write about them. The way we learnt with Peter was by being required to do an incredible amount of writing. We had to do a long essay every week, and I have never since had to write so much and under such pressure.

In the very first week of the lower sixth we had to read the six volumes of Gibbon’s Decline and Fall of the Roman Empire and write an essay on his historiography. Aged 15, that was an almost inconceivable task. I turned in five or six pages - a great deal more than I had written before - but Peter regarded it as inadequate.

He believed in the long essay as the most effective vehicle for historical analysis. He would never comment on style and composition (two, much younger, masters taught me how to construct essays), and his microscopic margin notes were entirely related to our historical argument and whether he agreed or differed with our interpretation of events.

When I went to King’s College, Cambridge, I got a first in part one of the history tripos almost exclusively thanks to the intellectual demands of Peter’s teaching. The Cambridge syllabus was broadly similar to A-level, and when it came to the exams I was able to write essays based on the work I’d done at Marlborough.

Peter was also my tutor, and it was in that capacity that my view of him as being solely a teacher gave way to a recognition that he had a life outside the classroom, a life based on a continuing interest in history and art history. I was not at all good at or interested in games, and at weekends I would retreat to the house on the edge of the playing fields which he shared with his wife, Betty. She was about 10 years older than him and ran an antiques shop in the town. They led quite independent lives.

Peter and I went on walks and he treated me as an adult. I found it remarkable that he was so unprotective of his weekends and didn’t mind my disturbing him. He didn’t have children of his own, so there may have been something paternal for him in our contact.

We never talked about art in aesthetic terms. He had no interest in visiting museums and galleries; he taught the Italian Renaissance but had never been to Italy. I, on the other hand, felt strongly that such visits were essential to one’s interest in art. (I already had a nastily precocious sense that I wanted to work in art history.) Such interest as he did have in the realities of art he satisfied on his annual holidays in Spain, by studying the sculptures on the pilgrimage road to Compostela.

After I left Marlborough we corresponded fairly regularly and I visited him often enough to maintain a sense of contact. I don’t know the level of pleasure he took in what happened subsequently to his pupils, who in-cluded Ben Pimlott, biographer of Harold Wilson and the Queen, although I do remember him mentioning Hugh Kennedy, who is now professor of medieval history at St Andrew’s University.

I last saw Peter about 10 years ago, when he was dying of lung cancer. He was propped up in bed like a medieval saint and died a day or so later.

Peter gave me an ability to digest and analyse information which has proved to be absolutely invaluable. I retain a sense of admiration for him as a person and gratitude for what he did for me - a debt I have not repaid as fully as I would have liked.

Charles Saumarez Smith, 43, art historian, has been director of the National Portrait Gallery since 1994. His selection of 120 of the gallery’s 9,000 works, The National Portrait Gallery, has just been published (NPG Publications, Pounds 25 hardback; Pounds 17.50 paperback). Gallery Week ends on July 27