Sir Nicholas Henderson spent 40 years as a confidential adviser to the British Government. His diplomatic career took him to Athens, Santiago Vienna and, as ambassador, to Poland, Germany, France and the US where he was instrumental in securing Washington’s support for Britain in the Falklands conflict. After his retirement he advised a wide range of private employers including Sotheby’s, Eurotunnel and the BBC.

No one less resembled the éminence grise of diplomatic service stereotype. In many of the things he did Henderson attracted attention and provoked controversy. Friends pointed to the sustained brilliance of his insights. Enemies hinted at flamboyance shading into the meretricious. Most agreed that “Nicko” was a buccaneer.

John Nicholas Henderson was educated at Stowe School and Hertford College, Oxford. He started his diplomatic career in 1942, at the age of 23, in the Minister of State’s office in wartime Cairo. By the end of the war he had moved to the Foreign Secretary’s private office in London, serving Anthony Eden, who went on to be a Conservative Prime Minister, and Ernest Bevin, the postwar Labour Foreign Secretary.

Henderson then had a series of overseas posts, in Washington, Athens, Vienna and Santiago, refining his sensitivity to what foreigners wanted and what they would stand. It was a skill that throughout his career balanced his almost preternatural instincts about ministers, Westminster and Whitehall.

By 1959 Henderson was back in the Foreign Office where, in particular, he became expert in Soviet relations. Then, at 44, he returned to the private office as Principal Private Secretary. The private office, close to power, knowing everyone, in an arena for influence and ingenuity, was perhaps where Henderson was most at home, and years later he anatomised it in an elegant memoir. He stayed there until he was transferred to the No 2 post in the embassy in Madrid in 1965.

In 1951 Henderson had married Mary, a Greek resistance heroine, who was a journalist by background and socially active by disposition — qualities that closely matched Henderson’s own. In Madrid Henderson’s intellectual and social curiosity, his gift for friendship with the interesting and the ambitious, as well as his energy and gregariousness, were displayed on a larger stage than had been available in earlier postings. They were seen to even better advantage when, at 50, he was appointed Ambassador in Warsaw.

It would not have been easy for any British ambassador to shine in the sombre Poland of the late 1960s and early 1970s. Gomulka’s 1956 glory had long since faded — he lost office in 1970, to be succeeded by the initially cautious Edward Gierek. There was more excitement in monitoring Poland’s Western links, but in Henderson’s time in Warsaw it was German diplomatic issues that stole the limelight.

The Warsaw Treaty occupied minds, with events such as that when Willy Brandt, the German leader, knelt at the Warsaw Ghetto attracting most attention.

Henderson served for three years in Warsaw before becoming Ambassador to the Federal Republic of Germany in 1972-75. His move to Bonn was a promotion to one of the Diplomatic Service’s half-dozen most-coveted prizes, and to the city that more than perhaps any other held the key to the success or failure of Britain’s European and East-West policies.

There were those who thought Henderson an opportunist but there was a commitment on substance too. At a bad time for Britain’s relations with the continental community, he fretted about whether he was really getting German views across in London. A strange comparison haunted him: with his own namesake and predecessor as Ambassador to Germany, Sir Nevile Henderson, who in the years before the Second World War so signally failed to avert catastrophe. Oddly enough, they had similar handwriting as well as a surname in common.

He went from Bonn to Paris as Ambassador from 1975 to 1979, and then to Washington. His valedictory dispatch from Paris won notoriety that some observers considered unfortunate, although others thought it astute. In it, as he approached the Diplomatic Service’s mandatory retirement age, he set out his concerns about Britain’s economic and social performance and about the failure, as he saw it, to position Britain in Europe in a way that would bring influence to bear on the development of the European Union. It was a heartfelt essay that concluded that Britain had become “poor and unproud” and set an example “not to follow”.

It leaked, The Economist printed it, and a political storm blew up. Many observers assumed that Henderson had at the least consented to the impropriety, with critics suggesting it was typical of the man. But whether because of the storm or despite it, Margaret Thatcher decided as she came into power that Henderson was the man she needed in Washington. So, scarcely missing a beat, the Hendersons translated themselves from the Faubourg St Honoré not into quiet retirement but to Massachusetts Avenue. He spent three brilliant years in Washington, culminating in bravura performances during the Falklands War in front of politicians and opinion formers on Capitol Hill and in the glare of television cameras.