Sir John Stevens, the Commissioner of the Metropolitan Police, was clearly taken a little by surprise by the turn of events this week. He knew that the royal coroner, Michael Burgess, was to ask him to investigate rumours that the car crash which killed Diana, Princess of Wales, was not an accident. But he had not been counting on a revelation by the Daily Mirror of a letter she wrote, 10 months before her death - without a shred of evidence to substantiate the claim - that such an "accident" was being plotted by the heir to the throne, Prince Charles.
The police chief's response was revealing. First he announced that an inquiry into her death would be conducted by Commander David Armond (who for months had been liaising with the coroner). But within 24 hours he changed his mind, and appointed a more senior officer, Deputy Assistant Commander Alan Brown. Mr Brown, it was said, would report to Sir John who would "oversee" the investigation after meeting the coroner to discuss its terms of reference.
The official reason for the change given by the Met was that Mr Armond - a leading member of the anti-terrorist branch's chemical, biological, radiological and nuclear command team, and clearly a man with plenty to do - had "other commitments". In reality it was an acknowledgement that things could now get very tricky indeed. It was also a demonstration of the political nous which has placed John Stevens where he is today.
With nearly 29,000 officers under his command - about a quarter of the country's police strength - the Met Commissioner is clearly Britain's top policeman. But Sir John Stevens has claim to such a title in his own right. Not merely by virtue of the 27 official commendations for outstanding ability or courage he has notched up in his 40-year police career. Nor because he has run the longest and most complex criminal investigation in British history - into charges of collusion by the security forces in murders in Northern Ireland. Not even because when he was appointed to the top Scotland Yard job three years ago he was variously described as "the best detective in the country", "a man with a CV to die for" and "a copper's copper".
What is striking about the plaudits for John Stevens is that they come from every direction, from fellow policemen, from politicians and from the public in the areas he has policed. Surprisingly perhaps for a policeman, when you ask around it is difficult to find anyone with a really bad word to say about him.
Another recent incident gives a clue as to his ability to keep everybody happy. After the débâcle at Windsor Castle, in which a comedian dressed as Osama bin Laden gate-crashed Prince William's 21st party, Sir John was swiftly on the scene. In private he was furious with the policeman in charge, Chief Inspector Mark Goddard. In public he defended his officers, telling the press: "When you are looking at something which has been a failure in totality, like this, it would be wrong to pick on any individual."
Meanwhile, behind the scenes, he acted. Mr Goddard was moved to undisclosed new duties by "mutual consent". A new security chief to concentrate solely on royal residences was appointed. And exercises, involving the SAS, to retrain the 400-strong royalty and diplomatic protection branches were initiated.
There had been those who had their doubts about John Stevens before he took over the Met. When he joined the police in 1963, after a middle-class boarding school upbringing, he twice failed to be selected for fast-track promotion through the Bramshill staff college. Tellingly, the reason given was that he was thought "too operationally minded". It was to prove a virtue, not a shortcoming.
His progress through the ranks in London, Hampshire, Cambridge and Northumbria was marked by a dogged commitment to investigating crime, covering everything from leading the hunt for the escaped spy George Blake to routine London murder investigations and masterminding an operation which revealed that half the British Airways' baggage handlers at "Thiefrow" were stealing from passengers.
Eventually he became a chief constable in Northumbria where he tackled inner-city crime and rioting in Newcastle and North Shields with remorseless vigour. He moved men from desk jobs on to the streets. Over the five years he was in charge, the crime rate fell by 42 per cent and the ram-raiding phenomenon in the North-east - using stolen cars to ram shops to gain entry - was virtually eliminated. When he left in 1996 he was appointed one of Her Majesty's Inspectors of Constabulary, which developed his wide range of contacts.
For all that, there were those who wondered if he had the political finesse for the top job. No doubt this 6ft 3in intimidating figure, fit from cricket, rugby and squash, was a "copper's copper". He was tough when confronted by terrible scenes: a baby with its throat cut, a receptionist blown to bits at the Hilton, an IRA bomb which slaughtered a military band.
He was brave. Once in Northern Ireland he was told that a group of loyalist terrorists had discovered where he and his team were staying, and were drinking in a nearby bar. He immediately walked up to the leader and said: "Good evening, gentlemen. Is there a problem?" The group left without finishing their drinks.
He was clubbable, once enjoying the nickname Captain Beaujolais because of his fondness for red wine and his bellowing bonhomie. He was swashbuckling; he owns a Cessna aircraft (his father was a pilot) and flew charity missions to Romania. And if he was notorious for his short temper and his "bollockings" of staff who fell short of his high standards, he was accessible too, frequently going out on patrol with ordinary PCs to keep in touch. Indeed, the lower ranks are noticeably more relaxed in his company than the top-brass sidekicks who more often see the explosive side of his temperament. But could he cope with the political sharks in Whitehall?
In the event, those with reservations have been surprised. His time in academia has helped. Three years as a mature law student at Leicester University after 10 years in the force "certainly changed my views", he said. "I was blinkered before I went there." Another big influence was his time in 1984-85 teaching comparative law and policing at New York's City University, where two-thirds of the students came from ethnic minorities. He listened in both places, and learned.
He has been helped too by the air of rectitude about him which in part is moral - he is a man of strong Christian faith - and part institutional. He was appalled when one former royal protection officer, Ken Wharfe, wrote a book, Diana: Closely Guarded Secret, revealing information he obtained while on duty. And at the end of the 14-year three-stage Stevens inquiry into collusion between elements in the security forces and loyalist paramilitary murder squads, he said: "I've been in policing for too long to be shocked by very much, quite frankly." But he added, "I think I could say that some of the things that we found have surprised me."
But what Northern Ireland revealed which surprised those who thought him a bombast was an unexpected political skill. Stevens handled his inquiries there with considerable delicacy. Despite personal threats made against him, and a mysterious fire which destroyed important documents at his inquiry headquarters in Belfast in 1990 (which Sir John had a hunch was arson by rogue elements in the Army), he kept his cool. Only when he became convinced that senior staff in the Ministry of Defence were blocking his inquiry did he go directly to see the Prime Minister; soon afterwards a large number of papers, which officials previously claimed did not exist, was suddenly available.
The interim Stevens Report, when it came last year, was unexpectedly hard hitting, concluding that collusion had indeed occurred - and ranged from "the wilful failure to keep records, the withholding of intelligence and evidence through to the extreme of agents being involved in murder". Some 43 convictions have since been secured and Sir John is due to return to Northern Ireland soon to ensure that his 21 recommendations on intelligence services operations have been implemented. If his report on the death of Diana is as direct it will undoubtedly upset one side or another.
All these skills have been in evidence in his three years as head of the Met. He has revitalised a force demoralised by the aftermath of the Stephen Lawrence case, restoring the enthusiasm of ordinary officers to get out on to the streets and catch muggers and burglars. He has improved crime figures, and is privately irritated with David Blunkett, the Home Secretary, for not acknowledging this.
He has won the argument over additional funding to send extra police officers out as beat bobbies in a conscious attempt to move back to the era of Dixon of Dock Green; reassuring the public is now "an end in itself", he says. He has also established a better relationship with the royal family after the collapse of the trial of the royal butler, Paul Burrell, making clear that, contrary to some press reports, the royals had "fallen over backwards" to help the police in the case.
But this last is a relationship which could now be tested again in the investigation demanded by the royal coroner. Over the 15 months the inquiry is expected to take, Sir John will have to decide how much of the 6,000 pages of evidence taken by the French inquiry into Diana's death needs to be reopened. He knows he needs to avoid, on the one hand, allegations of unnecessary intrusion upon the relatives of the dead, and, on the other, giving credibility to claims of an establishment cover-up.
He will also, presumably, have to decide whether - and how - he is to ask the heir to the throne such questions as "Did you order your ex-wife's murder?" and "Did you have anything to do with her death?". If someone has to do it, then probably Sir John Stevens is as well equipped as anyone. Whether it's the best use of his time and talent is another matter.
Born: John Arthur Stevens, 21 October 1942, son of an RAF pilot.
Family: Married to Cynthia, a district nurse, with two sons and one daughter.
Education: St Lawrence College, Ramsgate. University of Leicester (LLB), Southampton University (MPhil).
Police career: Joined the Metropolitan Police, 1963. Rose through the ranks; DS, police staff college, 1983-4; Assistant Chief Constable, Hampshire 1986-89; Deputy Chief Constable, Cambridgeshire 1989-91; Chief Constable, Northumbria 1991-96; HM Inspector of Constabulary 1996-98; Deputy Commissioner, Metropolitan Police 1998-99; Commissioner, Metropolitan Police 2000-present.
Inquiries: Headed inquiry into alleged malpractice at National Criminal Intelligence Service, 1994-96; inquiry into collusion between the RUC and loyalist terrorists, 1999; inquiry into the death of Diana, Princess of Wales, 2004.
Hobbies: Walking, cricket, squash, rugby, flying (qualified pilot).
He says: "If you believe what you read you would never go out of the door."
They say: "He's doing a fantastic job - although that's not to say he can't still be a bit barking at times." - senior colleague in the Metropolitan Police