John Rentoul, writing his biography, struggled to gain access to the `accessible' Labour leader
I must be one of the first political biographers to listen to rock music as part of my research. Not only did Tony Blair once snarl Mick Jagger's words in a band at Oxford, but he still listens to (mostly rather dreadful) rock music now.

Part of Blair's appeal as a politician is his apparent accessibility, the fact that he is recognisably a normal person, whatever the special qualities which make him an imminent prime minister. To me, as a middle- class, Oxbridge-educated father, born in the 1950s, he might seem particularly accessible. Certainly his values and cultural references seem less remote to me than, say, John Smith's or John Major's.

I decided to write a book about Blair soon after the 1992 general election, because I was impressed with him and thought he would be Labour leader one day. As a journalist, I knew him slightly. Making films for the BBC's On The Record, I had been to his house in his Sedgefield constituency and followed him to America when he went with Gordon Brown to talk to the victorious Clinton campaign team. But I only really began work on the book when Smith died in May last year.

The process of writing an anticipatory political biography - that is, of someone with no experience of high office - raises the question of how the author can really "know" the subject, because the "character" issue is central to any judgment about what kind of prime minister Blair would make.

Blair's normality and accessibility is inevitably a facade of some kind. This is not to say that he is insincere, merely that his true character remains largely unknown and unknowable. One of his teachers at Fettes College, David Kennedy, says: "He was so affable that you couldn't call him reserved, but you never saw his real self. He didn't like to expose himself in case someone spotted a weakness. Don't forget that he was a superb actor." After a year of close observation, I am still struck by the intricacy and technical sophistication of the public Blair, the best communicator in British politics.

When I told Blair I was writing the book, the week after Smith's funeral, his only comment was: "That's premature." I was not given access to his papers or allowed to interview members of his family, but he was as guardedly helpful as you would expect of a practising politician.

My limited access to him was governed by considerations of media management rather than history - it must be sensible for any politician to co-operate, without any suggestion of authorisation, with a profile writer or author so that they can present their version of contested events. Although when I discussed the book with him, his account of incidents where other sources offered contrasting interpretations was usually bland or unconvincingly sanitised. The truth of episodes such as his decision not to run for the deputy leadership in 1992 or his relations with Gordon Brown in a "secret leadership campaign" between Smith's death and Brown's withdrawal could only be approached by talking to as many of those involved as possible.

If there was any attempt to coach witnesses, it was discreet. Blair's office contacted his school within days of Smith's death to ask them not to release photographs or documents, and spoke to some of Blair's oldest friends. I was surprised when a university friend of his in Australia used distinctively Blairist terminology, such as "the left-of-centre" - but then Blair does keep in touch with them anyway.

His only other involvement in the book was to check some of the facts and, as the book neared completion, to ask, "What's your thesis?" every time I saw him, despite my repeated insistence that I did not think it was that kind of biography.

So I have formed a view about Blair's character not on the basis of direct personal knowledge, but on the facts of his life and the testimony of various people with whom he has formed lasting bonds of loyalty.

The critical event, it seems to me, was his confirmation in the Church of England at the age of 20, at the end of his second year at Oxford University. The importance of this event in understanding him can hardly be understated. He became a Christian and a socialist at the same time. He discussed with friends the idea of going into the ministry, but decided instead to pursue his ethical commitment to "make a difference" through politics.

Many of the modern anti-clericalists in the Labour Party, however, find Blair's religion a bar to understanding him. This may be partly because Blair is sometimes described as a High Anglican - presumably because his wife Cherie is a Roman Catholic. In fact, Blair's religion is non-denominational, leaning towards the pastoral rather than the ceremonial - although I am an atheist, his Christian socialism seems quite recognisable to me as the moral philosophy of the ethical socialist tradition. Blair's belief that people can only be fulfilled through a sense of "community" with others, while obviously some way short of a political programme, is clearly genuinely held.

Of course, such beliefs are not totally incompatible with political opportunism, although they do suggest that there is a bottom line to Blair's politics - which he drew himself in the new Clause IV of Labour's constitution.

Principle is pointless without determination. And Blair is certainly eager to give the impression of "strong leadership". But behind the self- conscious facade, he has already turned out to be a strong leader. He has yet to take a principled stand against public opinion - although, as he says, the distinction between principle and vote-chasing is partly a false one. The leaders who give electoralism a bad name, like Harold Wilson and Bill Clinton, are those who come to power by telling voters what they wanted to hear but who then fail to deliver in government.

The main weakness of Labour governments in Blair's lifetime has been the clash between what the party wants of the government and what the public wants. One of his reasons for changing Clause IV was to persuade the party to accept in advance the modesty of his ambition. That acceptance is not yet complete, and there is no doubt that as prime minister, Blair would offend many of his own party. But equally, there is every likelihood that, as his ethical socialist programme becomes better defined, he will build support for it among the public.

`Tony Blair' by John Rentoul is published by Little, Brown, pounds 16.99.