"There are only little bits of the original wall left," Ackroyd acknowledges, perched in an armchair in his Islington home with a bottle of wine and the first of many Silk Cuts. His slicked-back yellow hair, pinkish face and neat moustache give him the appearance of a colonial Sergeant Major. "But the new buildings are now effectively another London Wall. It's been resurrected in a different form. It happens all over London. It's always been ugly, a vandalised city, but I hope it stays that way because that's its nature."
While tourist boards would have us believe that London is "cool", planners strive to preserve the old and restrict the new, and most Tube-users dream of emigrating to the country, 49-year-old Ackroyd is celebrating the ugliness of swathes of the capital. But then he has never been a man to follow the crowd. His success as a writer - a string of best-selling biographies and novels which have garnered two Whitbreads, the Guardian Fiction Prize and the Somerset Maugham Award - is based on his highly personal and much-debated view of history.
For a start everything - including his new book, A Life of Thomas More - revolves around London. The capital is the place where Ackroyd finds inspiration in unconventional and often dark backwaters. It is not just the backdrop but a character, as vital as Inspector Morse to Colin Dexter.
Then there is Ackroyd's rejection of the prevailing English style of biography - chronicling dates, events, dynasties - in favour of a psychological intensity that had him, in his biography of Dickens, including fictional passages in order to bring his subject to life. The old school, says Ackroyd, is "completely worthless". To try, as it does, to judge historical characters is a fool's errand. "People will no doubt ask me about More `did you like him?', but the question never arises in my mind. Moral judgements at this late stage seem superfluous."
The zeal with which he preaches this gospel has caused great offence, but perhaps it is designed to. Certainly it has got Ackroyd's name noticed. And while some critics have, in the past, savaged his style, of late they have become more enthusiastic as it has attracted the ultimate flattery of many imitators. Academics meanwhile continue to mumble and groan, though their biggest gripe seems to be the reported pounds 1.25 million publishing deal that keeps Ackroyd in some splendour in one of the swankiest bits of fashionable Islington.
He is unrepentant. "There is the romantic myth that great writers shouldn't be concerned with money. It's nonsense - Dickens and Shakespeare spent their lives being concerned with money." His tone quickly reveals another of Ackroyd's alleged sins - he has a reputation for being prickly, arrogant and dismissive if you catch him in the wrong mood.
I always try to take the charitable approach to what passes for arrogance, namely that it is a cover for shyness. And it is certainly true that Ackroyd is a loner. It's tempting to think that it all dates back to when, as a clever, young boy, growing up in a council house in East Acton in the shadow of Wormwood Scrubs with his mother and grandmother, he would go off on his own exploring London. "From an early age," he once wrote in a comparatively rare moment of self-revelation, "the city became the landscape of my imagination. You don't have to be brought up in a grand house to have a sense of the past and I truly believe that there are certain people to whom or through whom the territory - the place, the past - speaks."
He is unabashed in making himself sound like a mystic. "If I walk down Cheapside," he says, "I can remember what it was once like. What I mean is I can visualise." Topography for Ackroyd is a spiritual device via which he can move backwards and forwards in time. In his fiction, there will be a figure from the past and one from today. In The House of Doctor Dee, a novel set on Clerkenwell Green, the two main narrators are a 17th- century alchemist and Matthew Palmer, a modern-day amateur sleuth. Ackroyd's latest journey takes him back to a period he hasn't covered before, late 15th and 16th-century London, to Milk Street in the City and the Chelsea Embankment, in search of Thomas More.
For Catholics More is a martyr who lost his head because he refused to put Henry VIII's needs before the Pope's. For lawyers, More was a principled Lord Chancellor who sums up the mindset of their profession. And for liberals, he epitomises the man of conscience who preferred to die rather than bow to a tyrant.
Ackroyd, of course, comes at it from a different angle. More for him is first and foremost a Londoner, one who spoke the local dialect and who lived his life within the city. In that sense he links neatly into the chain. But the connections go deeper. For More shares, in Ackroyd's portrait, one overriding characteristic with some of his previous subjects like Charles Dickens and William Blake. All were, he says, Cockney visionaries.
I am struggling mentally to get beyond chirpy Cockney sparrows in the Barbara Windsor mould when I notice that an embarrassing silence has descended on the smart, uncluttered and bookless sitting room. Sensing my unease, he laughs the most wonderfully reassuring roar which breaks the ice. Even if it is only because he feels sorry for me, at least now, when we resume on the Cockney visionaries, that odd mixture of aggression and defensiveness has been replaced by warmth.
Blake, Dickens and More, Ackroyd says, "all shared a strange imaginative reaction to London which became for them a very potent symbol of human life itself. It shaped their whole reaction to the world." In More's case, Ackroyd shows in his biography that this Catholic saint took the city where he lived as the basis for Amaurotum, capital of the island state of Utopia, subject of his celebrated satire of 1516. "Amaurotum is," Ackroyd writes, "London redrawn by visionary imagination, a pristine city in which ... there is no greed or pride or disorder."
The particular quality of an Ackroyd book comes from the pains he takes to understand his characters. "With Blake, I had to become a Blakeian, with Dickens, Dickensian, and with More, I wanted to intrude myself into this very strange early Tudor imagination." This Ackroyd achieves, in a flourish reminiscent of his fictionalised passages in Dickens, with a series of set-piece medieval liturgies at key moments. A baptism, a marriage and the long stretches of private and penitential prayer favoured in this period are all recreated in exact detail as if from inside More's head in an attempt to help secular 20th-century readers to enter the mind of a man who saw no tension between his religious and his political duties.
"Do you think they work?" he asks nervously. "I wasn't very sure about the baptism at the start. People might get no further." It is said with no false modesty and reveals an attractive openness and surprising insecurity in Ackroyd. I say what I honestly think - that the descriptions of the rituals are one of the most powerful and helpful parts of the book. He looks reassured.
The once strict dividing line between fiction and non-fiction needs, in Ackroyd's opinion, re-evaluating. "I think conventional biography is coming to the end of its natural life and I'm bored with fiction. I want to merge the two. My next book is a biography of London, written as if it is a person. It's neither fiction nor non-fiction. It's somewhere in between."
With More, Ackroyd says, "the challenging is to consolidate him as a political leader and also as a religious figure. On the one hand he was whipping himself in his library during prayer and on the other he was engaged in the elaborate rituals and decisions of being Lord Chancellor. For More they were the obverse of the same coin but for us, trying to understand a character like More is like monkeys trying to figure out TV. It was a different civilisation. The secular permeated the religious in all its aspects. One of the points of the book is trying to recreate this Catholic European civilisation of which More was a part."
Isn't he worried that his particular slant on More may offend many Catholics? Ackroyd has a problem with the whole notion of sainthood. He doesn't believe in it - but even if he did, he adds, then there is much in More's life, as opposed to his martyr's death, that would militate against sainthood. "He was very sharp-witted and quite caustic and had an innate sense of superiority which he tried to keep in check but sometimes couldn't. And in his life he amassed a fortune and was even in some senses a property speculator."
There are enough parallels between Ackroyd and More to make me wonder out loud if the man for all seasons had sounded a personal note with Ackroyd, who lapsed from the Catholic Church at 15. "Being an English Catholic, I'm sure one touches somehow on the Catholic past. It must be part of you still in some peculiar way." Again, there is that transcendent, mystical view of history.
Among Ackroyd's future plans - one subject often prompts another, so as Blake wrote two poems called "Milton" and "America", Ackroyd followed his biography of Blake with a novel, Milton in America - is a book about the English imagination which will identify what he calls a "buried Catholic sensibility or culture". This still exists on these shores, he says, but is largely below the surface, glimpsed only in our appetite for spectacle, display and ritual. "If you think about it, England was Catholic for 1,500 years and Protestant for just 500, so where does the balance lie; where did it lie, for instance, in the almost neo-Catholic reaction to the death of the Princess of Wales?"
I am rebuked, though, when I gently suggest that Ackroyd is turning to religion. "And all my books have been religious in that they're all concerned with that great divide between those who have a secular view of the world and those who have a religious sense." He places himself in the latter category but laughs at the idea that he might return to a Mass-on-Sunday-do-what-the-Pope-says style of Catholicism. The prospect of listening to endless papal pronouncements on homosexuality - Ackroyd's literary assistant and partner of 22 years, Brian Kuhn, died in 1994 - can hardly be enticing.
Ackroyd clearly retains some affection for organised religion, whatever its shortcomings. After all the London landscape would be very different without the churches - especially those created by Nicholas Hawksmoor - that feature in his books. "It would be nice," he admits with a twinkle in his eye, "if one could create one's own religion. I think I might call it" - and it is now plain that he has given this some thought, if only to develop the joke - "the Church of Past Times. But it's very difficult and very expensive these days, so I write books instead."
8 `The Life of Thomas More' by Peter Ackroyd is published by Chatto, pounds 20