Charles Moore tells me quite an eerie story before we get on to discussing Margaret Thatcher’s relationship with Indira Gandhi — the two Prime Ministers who recognised that politically they had little in common and yet developed a close bond.

On April 8, he was on the train from his home in Sussex, editing the final proof pages of his biography of Margaret Thatcher. When he got to his lunch appointment in London, Moore was given the news flash that Lady Thatcher, who had been staying at the Ritz Hotel in London since an operation in December, had died after suffering a stroke that morning. Moore apologised to his hosts and rushed off to do radio interviews and write an assessment for The Daily Telegraph.

By a remarkable coincidence, Moore realised later that she had died almost exactly at the same time as he had finished correcting the final proof pages. It was 11.30am.

“It was rather odd,” he remembers, as we chat at the Telegraph office in Victoria. “I was finishing off the final proofs of my book actually on the train and I had indeed finished the last page of Volume One. What I now realise is that it was almost exactly the moment when she died. I had literally finished the very last page.”

Moore was first chosen by Mrs Thatcher to be her official biographer in 1997. His contract from Penguin came through in 1998 but since he was editing The Daily Telegraph at the time, he could devote only a limited amount of attention to the book. The understanding was that in return for access, the biography would not be published in Lady Thatcher’s lifetime.

When her husband, Denis, died in 2003, Moore attended the funeral and realised she was in a bad way. He resigned as editor in the autumn of 2003 and began to work in earnest on the biography from early 2004.

For anyone in India tempted to write a well researched, big biography, Moore’s book serves as the perfect model. Margaret Thatcher: Volume One: Not For Turning (Allen Lane; £30) is 859 pages and covers only four years of her reign as PM. The remaining seven-and-a-half years have to be accommodated in Volume Two which won’t come out for another few years.

“In this book there are 315 people interviewed so that means more than 315 interviews because quite a lot of people were interviewed twice,” he says. “And some of the people were interviewed for both volumes, of course. So I think by the time the whole thing is over about 500 people will have been interviewed.”

Moore tape-recorded all his interviews but he soon stopped transcribing them since he considers such an exercise “a tremendous waste of time because people say so much nonsense”. Instead, he relies on careful notes and writes with all his notebooks spread out in front of him.

He has been through Lady Thatcher’s massive archive deposited with Churchill College, Cambridge — they detail Swraj Paul’s meetings with Mrs Thatcher on Indira Gandhi’s behalf — as well as government papers.

Although it is said that journalism represents the first draft of history, Moore believes there is a distinction. “Journalism means daily stuff and history means a full account of what has happened in the past — this is a book about a person whose career ended a long time ago and whose life has also now ended. And I am the first person to see all this material and in that sense I am the first person to give it a historical perspective rather than an argumentative or journalistic one.”

He has, of course, had to omit a great deal of material.

“All history is a matter of selection,” he observes.

Sometimes, at four in the morning he worried whether he could ever pull together all the material into a book. Later, he worried whether he would ever finish the task, which he says is still not completed. But most reviewers recognise that Moore’s biography, which is both well written and readable, is also a work of fine scholarship.

Mrs Thatcher wouldn’t give her papers to Oxford which denied her an honorary degree although she retained cordial links with Somerville, her alma mater. This was also Mrs Gandhi’s Oxford College.

“That’s another reason why there is that fellow feeling (between Mrs Thatcher and Mrs Gandhi),” Moore points out.

One assumes Mrs Thatcher picked Moore as her official biographer because she knew he was sympathetic without being “part of her gang”. After Eton, he had read history at Trinity College, Cambridge. He knew the political milieu well because he had edited The Spectator magazine from 1984 to 1990, The Sunday Telegraph from 1992 to 1995 and The Daily Telegraph, for whom he still writes a weekly column, from 1995 until he stepped down in 2003.

After Lady Thatcher’s death, there were last minute corrections to be made and the acknowledgements and the index to be finished but the biography appeared on April 23. By the time I get to see Moore, I am advised he has already done 40 interviews.

“Definitely more than that, I would say 100,” he corrects me, looking surprisingly relaxed.

Given the global interest in “the Iron Lady”, there has been a pattern to the questions he has been asked — “the question of her as a woman, legacy question, style of leadership question, the divisiveness question; people are very interested in her as a mother, and her as a wife and my own relationship with her”.

Since the book covers Mrs Thatcher’s special relationship with US President Ronald Reagan, Moore is off to America shortly for a media blitz in New York and Washington, possibly followed later by a more extensive tour of the United States.

What about India?

“I’d love to come to India, it would be particularly interesting,” he responds. “I have my Daily Telegraph duties and I must write Volume Two. But India would be high on my list of priorities if I could.”

As for Mrs Thatcher’s friendship with Mrs Gandhi, Moore goes on, “it certainly was a real thing with her — I may say a little more about this in Volume Two. My own opinion — and I must be careful because I haven’t done a lot of work on this — is it did not have a high content in political terms. I think it was more of a female solidarity, each recognising in the other some shared qualities which they admired. On the whole Mrs Thatcher is accused of not having much solidarity with women but I think she was interested in Mrs Gandhi’s situation and the parallels with her. And also they liked to talk about problems with their children. That (information) came from her entourage. Famous people usually have problems with their children.”

Moore emphasises that the Brighton bomb on October 12, 1984, when the Irish Republican Army (IRA) very nearly managed to kill her and half the Cabinet, “was another point of sympathy” between the two women. “This was a very big thing in Mrs Thatcher’s mind.”

In any case, after she would not concede political prisoner status to IRA hunger strikers, she knew she was the number one target. “She was always under threat but particularly after Brighton, it was extreme, extreme threat.”

She was “very struck” by Rajiv Gandhi’s assassination in 1991 and “feared it for herself”. His death preyed “particularly strongly on her mind”.

Mrs Thatcher was broadly sympathetic to the hardworking and enterprising Indian community in the UK but also wanted strict controls on immigration. In 1978, she outraged liberal opinion by saying during a TV interview that “people are really rather afraid that this country might be rather swamped by people with a different culture”.

Moore concedes: “There is always a slight tension between saying, ‘Immigrants are great but we don’t want too many of you,’ but that was, in fact, her position. She did not want white ethnic purity or any such nonsense but she did want a coherent and harmonious society which had a distinct British culture.”

In her youth, Mrs Thatcher had even toyed with the idea of joining the Indian Civil Service. Moore draws attention to a footnote in his book about a shadow Cabinet meeting in 1976 when Lord Carrington, later her foreign secretary, made a slightly disparaging remark about Sikhs. The Commons was debating whether Sikhs should be given special dispensation to wear turbans instead of crash helmets on motorbikes.

“Mrs Thatcher said sharply, ‘What did you say?’ Carrington said, ‘It was a joke, Margaret,’ and explained. She replied, ‘Well, it’s not very funny. These people fought for us in the war.’”

Moore tells me: “She had a particular regard for the contribution of the Indian subcontinent to the two world wars.”