What does it take to be a great foreign secretary?
What makes a great foreign minister? Some of those who have held the great office of state, including Lord Carrington, David Owen and David Miliband, and some of the ambassadors who had to work with them, talked about the stresses and strains involved.
The job of foreign secretary is unique in the British cabinet. He or she, Janus-like, looks out at the wider world and yet has to stay connected to British politics.
Though their time may be spent in departure lounges and foreign capitals, any foreign secretary who becomes cut off from Britain by constant travelling or, at the other extreme, stays in London and spends insufficient time on foreigners, is a failure.
It is difficult not to be overawed walking up Gilbert Scott's grand staircase and into the tennis-court-sized office of the foreign secretary on the first floor of the Foreign and Commonwealth Office, overlooking St James's Park.
The building was constructed precisely to overawe foreigners, and it is difficult not to feel small beneath the 10ft-tall painting of an Indian nabob that Lord Carrington had hung on the wall.
This is a place where it is easy to feel that you are in the company of the ghosts of Palmerston, Castlereagh and Canning.
I remember Robin Cook sinking into the sofa on his first day in office in 1997, looking round the room and saying to me: "Pretty good for a former Workers' Educational Association lecturer, eh, Jonathan?"
Politicians tend to think of the Foreign Office in terms of the old wartime joke that Margaret Thatcher used to delight in retelling. The man walking down Whitehall in the blackout asks the policeman which side the Foreign Office is on, and the policeman says: "The other."
"Exactly," she would say, according to Charles Powell, her foreign affairs private secretary.
Even some of the Foreign Office's top mandarins accept that ambassadors do on occasion go native, spending more time empathising with their host country than championing the interests of Britain.
The former Labour Foreign Secretary David Owen complained that the FCO, and particularly his permanent secretary, were too pro-European.
But most former foreign secretaries sprung to the institution's defence.
Lord Carrington said the problem was that people form the opinion that "you shouldn't really have much to do with foreigners, foreigners are thoroughly unreliable, you don't want to get on with the foreigners, and if you do get on with them, you're selling out to them. It doesn't seem to have occurred to them that surely it is better to get on with people than to quarrel with them."
Increasingly, the job of foreign secretary is to build personal relationships with fellow foreign ministers and do them little favours so they will support him - or her - in return.
Robin Cook used to buy large costume jewellery brooches for US Secretary of State Madeleine Albright every Christmas.
Some of these relationships turn into firm friendships, as did that between Jack Straw and Colin Powell, the former US Secretary of State.
In Europe, David Miliband complained about an excess of kissing if you are to remain friends with all your opposite numbers.
Other key skills might seem counter-intuitive.
Should the foreign secretary be able to speak foreign languages?
Of recent foreign secretaries, only Douglas Hurd did, having mastered Italian, French and Mandarin.
He says the Italian helped him build an alliance with Rome's Foreign Minister Gianni De Michelis, when Britain was being excluded from the Franco-German alliance.
Each foreign secretary takes a different approach to the job.
Sir Sherard Cowper Coles, the principal private secretary to Robin Cook, said his charge studied his papers like "the school swot" on the plane out to European council meetings.
Sir John Kerr, who was a permanent under-secretary at the Foreign Office, said Sir Geoffrey Howe and Jack Straw, who were both lawyers, would prepare as if they were getting ready for a day in court, while Lord Carrington would adopt a more languid style, pretending he knew nothing whatsoever about the subject until his victim had walked into an elegant trap.
As the role of the prime minister in foreign policy has grown, with modern communications and the proliferation of foreign policy aides to presidents and prime ministers around the world, so the role of the foreign secretary has shrunk.
There is an inbuilt institutional tension between Number 10 and the FCO that can be made worse by bad personal relations.
Charles Powell described the agonising weekly sessions between Sir Geoffrey Howe and Margaret Thatcher, where Howe would start on the subject matter in the middle and try and work his way back to the beginning, but she would lose patience and interrupt with her views after less than a minute.
But when foreign secretaries get on with their prime ministers, as Lord Carrington did with Thatcher or David Owen did with Jim Callaghan, then things can go smoothly.
The influence of the British foreign secretary has been further constrained by Britain's diminishing role in the world, the advent of a coalition government and the creation of a European foreign minister.
As Lord Carrington told me: "For somebody like me who's very old and was brought up in the days when most of the world was coloured red [the hue of the Empire in atlases], I find it very sad and humiliating that we aren't the great power we used to be. But it's no good being nostalgic about it."
A British foreign secretary can no longer throw his or her weight about. Their task now is to build alliances and persuade others to support our position.
So who has been the greatest foreign secretary of all?
When I asked my interviewees for their nominations, I got some surprising responses.
Jack Straw and Sir John Kerr nominated Geoffrey Howe for his steady, calm, lawyerly approach.
David Miliband nominated Tony Crosland, who held the job for less than a year, and David Owen plumped for Ernie Bevin.
Lord Carrington nominated Alec Douglas-Home and Douglas Hurd went for Viscount Castlereagh.
But perhaps the answer that most symbolised the changing roles was Charles Powell's, who nominated as the greatest foreign secretary Margaret Thatcher.
Jonathan Powell interviewed five of the nine surviving former foreign secretaries about their experience of the job for The Art of the Foreign Minister, which is broadcast on Tuesday 14 May at 20:00 on BBC Radio 4. Or catch it later on the BBC iPlayer.