In The State Within, the BBC television series, the British ambassador to the United States is a lone hero who unmasks layers of conspiracy at the heart of the war on terror and confronts the bellicose US defence secretary with the stirring words: “This war is illegal . . . Stand the planes down!” It is quite a lot to live up to for Sir David Manning, our man in Washington, a brainy, softly spoken career diplomat who gamely invited the screenwriters to lunch at his handsome residence, designed by Sir Edwin Lutyens in the late 1920s, so they could get a feel for his job.

The British ambassador has been quintessentially in the shadows yet absolutely in the know since serving as Tony Blair’s foreign policy adviser in Downing Street in 2001. Two years later he arrived in Washington, where he prides himself on his top-notch contacts and total discretion. He rarely offers himself for public scrutiny, feeling it is best left to elected politicians.

Manning’s flamboyant red-socked predecessor Sir Christopher Meyer scandalised Whitehall by publishing an irreverent, gossipy memoir, DC Confidential, which revealed that he had been sent to Washington with the memorable instruction “to get up the arse of the White House and stay there”.

It was surely a more comfortable place in the aftermath of the September 11 attacks, when America had the sympathy of the world, than it is today. When I meet Manning in the embassy’s elegant drawing room, Blair has just been receiving a bashing from Chatham House, a leading think tank, for getting nothing out of the special relationship with America.

The decision to invade Iraq, the report charged, was a “terrible mistake” that would oblige Blair’s successors to “rebalance” Britain’s foreign policy between Europe and the US. The Conservative leader David Cameron joined in recriminations, claiming the Iraq war had made Britain more unsafe.

As the Downing Street memos published by The Sunday Times in 2005 revealed, Manning was a cool enough analyst in the run-up to the Iraq war to warn Blair that the Bush administration “may agree that failure isn’t an option, but this does not mean that they will avoid it”.

During Manning’s period as ambassador — he is due to retire next year — that prediction has come uncomfortably close to the mark. With Blair suggesting last week that Iraq and Afghanistan could become holiday hotspots, Manning seems positively drenched in realism, but he has seen enough of the prime minister at close hand to know he is a true believer in Britain’s alliance with America.

“You’ll find within this relationship that there are strains and differences, but he’s never seen it as, ‘Now if I do this, what will Bush give me?’ That’s not the way Blair operates,” Manning says. “There are real consistencies about the way he has dealt with the world.”

Manning chanced to be flying into New York on the morning of September 11, 2001, and was astonished to see huge plumes of smoke emerging from Manhattan. He ended up in a “dive” of a hotel in Queens and the next day he toured the city. “It looked as though there had been a nuclear attack, because the cloud was like a pillar and sort of mushroom-shaped,” he recalls. “It was really terrifying. There is nothing like seeing it for yourself to understand why 9/11 is so seminal to what has happened in America.”

After the relaxed post-communist 1990s, it was as if “history started again in a rather unpleasant way”. He feels that, “whatever you say about the Iraq war, we were involved with a very, very serious terrorist threat before anything happened there”.

Manning’s experience as an adviser in Downing Street helped him to get a grip on the massive embassy in Washington — a shadow version of the British government that he calls “Whitehall on the Potomac” — and made him an early friend and ally of Condoleezza Rice, then national security adviser.

The pinnacle of the special relationship came when he hosted a surprise 50th birthday party for Rice at the embassy a few weeks after Bush’s second election victory in 2004. All of Washington society was there, including the president. “Everybody joked afterwards that if we surprised her it was dreadful, because no national security adviser can afford to be surprised, but I hope we did.”

Manning, 57, sees less of Rice today, not least because she is on a plane every other week as secretary of state. Yet, as the popularity of Bush has nosedived, there is a growing sense of detachment between Britain and America at every level, leaving Blair increasingly isolated as the last man propping up a tired affair.