The bride and groom are framed in the entrance of the church. Peter Ackroyd frowns. "How inconvenient. Are they getting married or not? I must show you where the Virgin Mary was supposed to have appeared. This church, St Bartholomew's, is one of the oldest churches in London. Late 13th century." We take five more steps. Ackroyd squints at the back of the bride, a billowing meringue of white satin and tulle, obstructing our progress. "She looks like a waxwork," he whispers archly.

A minute passes before Ackroyd springs forward. "Come on," he says walking straight past the stationary bride and groom. "Now where is Mary's chapel?" The groom looks at us, stunned. Surveying the shabby, beautiful interior, Ackroyd whispers: "St Batholomew petitioned the king to build this church. All the local children brought stones and built its foundations. Mary appeared as a vision in the 19th century. She told a monk, 'You must pray for me', but it never became a site of pilgrimage."

His gaze shifts to the nearly empty pews. "How odd. A wedding with no congregation." The organ starts playing. A vicar approaches, looking embarrassed. "Would you mind leaving? There's about to be a wedding." Ackroyd briskly responds: "Where's the chapel dedicated to the Virgin Mary?"

"Over there," points the dumbfounded vicar, "but you'll have to come back another time." Ackroyd nods and walks towards the exit, past the bride and groom, unabashed. "This is what appeals to me," he says, with a flamboyant sweep of hand. "Little bits of London that survive."

The capital in its countless incarnations, from Italianate Xanadu to medieval cesspit, has been central to many of Ackroyd's prize-winning novels, non-fiction works and biographies. Now, with the 800-plus page London: The Biography, he aims to endow the city with a living memoir, a painstaking work to remind us that the capital's history is alive and vibrant behind walls, beneath pavements and in the foundations of houses. Ackroyd's history, however, is not chronological, but linked by theme and laced with precise and pungent detail.

"Isn't it beautiful?" he asks as we stand outside St Bartholomew's, with its walled green. "Look at the stone (grey, muddy brown, odd bits of shingle). We're in the centre of the city, yet it's so quiet." The church, as everywhere in his favourite area, Clerkenwell, is imbued with the past. On Clerkenwell Green (actually rather a built-up, if leafy square), he points out the Marx Memorial Library. Walking slowly and speaking in staccato bursts, the author provides an engaging running commentary. Did I know that London was first named Londinium by the Romans in 43AD? That Wat Tyler had invaded Clerkenwell? That in 1816 Henry Hunt, one of the leaders of the Chartist movement, spoke to a crowd of 20,000 above the Merlin's Tavern just north of the Green? That the Tolpuddle Martyrs convened on the Green in 1837?

"London is a remarkable echo chamber," he says above the roar of clanking building work. "It's a visible, material relic. It is one of the few cities in the world which retains its archaic characteristics and still aspires to the future."

He smiles as he looks up at a new, four-storey office block, adorned with plaques naming 18th-century clockmakers: John Cranfield, John Moore, Sam Hutt. Incongruous? "No," says Ackroyd brusquely. "It's the old and new in embrace."

Then to Smithfield, home of the meat market since early medieval times, with Ackroyd darting left and right to look at buildings. He points over the gentrified cobbles: "The disabled lavatory is where the the martyrs were burnt." His book further details how in the 12th century the wives of Billingsgate traders were known as "fish fags" (from which we get fishwife).

In one chapter, he analyses the capital's history of hanging. In the late 18th century landlords let rooms with good vantage points to the highest bidder. Ackroyd also evokes the capital's subterranean labyrinth of pipes, old trainlines and catacombs. "One of the characteristic drawings of the city is that of its horizontal levels, from the rooftops to the caverns of its sewers, bearing down upon and almost crushing one another with their weight."

Ackroyd's fascination with the capital began early. He was born and brought up in Acton and as a boy he was taken to walk round the streets of Cheapside, in the heart of the city, by his grandmother, Catherine. Through researching his books, he slowly acquired a coherent picture of the capital's historical geography.

"London will always retain an element of mystery," he says, gazing thoughtfully at Smithfield's sunken Roman road. "It is continual revelation. Even if you venture down streets you've known all your life you see things startling or odd. This is a city built on money, power, trade and commerce. It's uniquely brutalising and ugly. It's built on the imperatives of money, not the need of its citizens."

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