Long after his father’s death, Sir Ian Blair’s mother told him that she had seen her husband cry only once. It was the day that young Ian had left them to join the police.

After all the sacrifices his parents had made to enable their son to go to Oxford to read English at Christ Church, his father was bitterly disappointed that his son had chosen to become a copper.

On first hearing of his intentions, when he returned home to Cheshire, his mother had blurted out: “What will we tell our friends?”

Sir Ian, however, has always suspected that there was more to his dad’s tears than thwarted ambition or snobbery.

“He was very fond of me and thought it was a difficult profession to go into. I suspect that he was concerned about the kind of horrible sights he was sure I would encounter.”

But even the perceptive Mr Blair could never have guessed the savagery and atrocities that lay ahead for his son.

Even if he could have seen his lad one day becoming Britain’s top cop, rising through the ranks to become commissioner of the Metropolitan Police, he could not have imagined the London tube and bus bombings of July 7, 2005, that his son would one day face.

The father would surely have wept harder, if given foresight to see the fate of the innocent Jean Charles de Menezes at Stockwell, a tragedy that appears destined to haunt his son’s days and nights for the rest of his life.

Thanks to the arrival of Boris Johnson as Mayor of London, Sir Ian has had ample time to reflect on his parents and all these things. It is almost a year to the day since he left the Met, and has been busily putting the literary skills acquired at Oxford to use by writing Policing Controversy, an account of his life and modern policing.

Over many years, including his time as commissioner from 2004 to 2008, Sir Ian and his family have lived in Oxfordshire. He was reacquainted with Oxford 15 years ago having been appointed as an assistant chief constable at Thames Valley Police, based at Kidlington, and has lived here ever since.

Moving up to Oxfordshire from the capital had been a tough decision, with his wife a successful lawyer running her own firm.

But the priority was to ensure the family stayed living together and by the time his children were in secondary education, the roots were too deep to uplift, even for the top job in policing.

As things have turned out, he will soon again be focusing his mind on crime in Oxfordshire.

He has agreed to become chairman of the Thames Valley Partnership, the crime prevention organisation that aims to cut crime in Oxfordshire, Berkshire and Buckinghamshire.

“Having shed the uniform and responsibilities of dealing with the consequences of crime, I am now eager to bring my experience to preventing crime in the first place, for the benefit of every community in the Thames Valley region,” he says.

Just how he came to shed the uniform still clearly rankles and a fateful meeting with the new mayor, who forced him out of the job he clearly loved, is the starting point of the book.

Ironically, the police chief, judged by his critics to have been too close to New Labour, found himself the victim of new powers given by Labour to the London mayor, which allowed Johnson to assume chairmanship of the Metropolitan Police Authority.

“There is no easy way to say this, Ian,” the former Henley MP had told him, before going on to suggest his departure would be best “for the Met, for London, and, frankly, for you”.

“I think what Boris did on the day was utterly unprecedented,” Sir Ian told me. “There are methods of removing chief officers through disciplinary processes. Nobody has ever been moved out on a whim before. I believe it is a dangerous precedent.”

So if he thought a dangerous bridge had been crossed, why did he not stay and fight what some were calling a Tory takeover of the Met?

“Well I could have said, ‘how very interesting, but I’m not going’. But I believe that would have put the service on a complete collision course with its police authority. I felt it was my duty to step aside and save the force from that kind of damage.”

If his time in the top job — leading a force of 53,000 officers and managing a £3bn budget — was cut short, July 7 means he is destined to remain one of the most recognisable police chiefs in history.

When the bombs went off he had been in his office at Scotland Yard, writing a speech he had to deliver that day. The first report to reach him suggested a power surge. As an image of the number 30 bus appeared on a screen, it was apparent that London faced something altogether more serious.

Sir Ian’s 16-year-old son Joshua had been doing work experience on the Sun that day and had rung earlier to say his train had stopped at Victoria Station. “I told him to get a bus. As soon as I saw the bus at Tavistock Square, I went cold,” he recalled.

“At that very moment when I had just begun to relaise that I was facing the greatest policing challenge of my life, I did not know where my son was and I might have just sent him to his death.”

He finally got through to Joshua’s mobile to tell him to get off the bus and walk to Scotland Yard.

The nightmare had to be relived again on July 21, when his staff officer walked into his office to inform him that there had been more explosions. But rather than casualties, there was to be a race against time to find the bombers.

“It was the most desperate manhunt in living memory. Nowhere else in the Western World had seen a group of suicide bombers go on the run. We knew there was a bomb factory and we had to assume anyone who had reached the psychological point where they were prepared to kill themselves, would stay at that psychological point.

“We had to assume they would go back to the bomb factory and attack again. There were some wonderful pieces of police work done. Most people will remember the arrest of the two bombers in West London.”

But there is also the memory of the mistaken killing of Jean Charles de Menezes by his men. In his book, Sir Ian risks further controversy, by including a tribute to the two officers who shot Mr de Menezes (referred to by their call signs as Charlie 2 and Charlie 12), who were told earlier this year they would not face charges.

He reflects: “Given what they thought they were dealing with, Charlie 2 and Charlie 12 in running towards and getting within a few feet of a suspected suicide bomber, together with the surveillance officer, who sprang on him and pinned his arms to his sides on the train, should each have been awarded the George Medal. Instead, and I have met all three, they live for the rest of their lives with the knowledge that they took part in the killing of an entirely innocent man.

“Had he been a suicide bomber and they had not shot him and the train had blown up, then, if not dead themselves, they would have faced an investigation for manslaughter.”

There is praise, too, for Cressida Dick, who was brought up and educated in Oxford and spent three years as area commander at Oxford, who was in command at the time of the Stockwell tragedy.

“I have rarely worked with a more professional and competent officer than Cressida Dick, who is widely regarded as one of the finest officers of her generation, with extensive experience of firearms operations.

“The key issue was identification and she and her team were convinced that Jean Charles had been positively identified. She faced a dynamic, rapidly-changing operation in a timeframe that was diminishing quickly, with a genuine risk of catastrophe.”

The events of that day enter his mind most days of his life, he tells me, bringing the subject to a close with the words: “I hope nothing like it happens again. But something will happen again.”

We are sitting in a cafe just a few yards from the university careers office, where a visit set his life on such an unexpected course.

He had arrived at Oxford wanting to be an actor. At university, he took to directing and was responsible for Christ Church’s offering in a college drama competion, a little known play by Alf Garnett’s creator, Johnny Speight, entitled If There Weren’t Blacks, You’d Have to Invent Them. He was to realise that he had no future in drama, when, in the final, he saw the offering overseen by one Mel Smith.

Joining the police, he now judges, was an act of rebellion.

“The other thing was that I couldn’t think of an acceptable reason why I shouldn’t try it.”

As he gets up to leave, he points out that I had not asked about what he views as his greatest policing achievement. I dutifully write down: “The introduction of community policing to London. The Safer Neighbourhoods all came out of that.”

But there is a final command. He does not want to be presented as an angry and bitter man.

“That would simply not be true,” says Sir Ian, a fine actor still, I suspect. But I believe him.