William Beckford's outrageous fortune

William Beckford occupies an odd niche in cultural history. Imagine the various preoccupations of Charles Foster Kane, Howard Hughes, Charles Saatchi, Oscar Wilde and Joris-Karl Huysmans in one human being, and you would only approach the manic, acquisitive, Homeric eccentricity of Britain's richest art collector c1800.

He was a lonely child who came into a vast fortune and spent years indulging his hedonistic whims. But he was also a man of genuine discernment, who put together the finest decorative-art collection of the early 19th century, who designed the finest Gothic-revival building in England, Fonthill Abbey, who brought the Gothic novel to a new pitch of scary Orientalism in Vathek, an Arabian Tale published in 1786, and who brought a new poise and style to English travel writing. He was also a skilled composer (aged five, he had received piano lessons from Mozart – "that moonstruck, wayward boy" – then aged eight); and an impresario of design with a fascination for gemstones and a virtual obsession with silver-gilt.

To say he caught the imagination of the Georgian fin de siècle needs qualifying. He was the most talked-about man of his time, but what they said about him was rarely complimentary. He acquired an early reputation as a feckless sybarite; then as a wicked paedophile; later as a misanthropic hermit; latterly, as a crabby old recluse. But when an announcement was made in August 1822, that Fonthill and all its contents were open for inspection with a view to sale, no fewer than 7,200 visitors came along (paying the equivalent of £55 for the ticket and catalogue) to gawp at the furniture, the rugs and jugs and bowls and caskets, the crazed profusion of beautiful objects gathered from the courts of the mighty, all over the world.

Some of those contents can be seen at Dulwich Picture Gallery, where the exhibition, William Beckford 1760-1844: An Eye for the Magnificent, is running until 14 April. It's odd to find an exhibition devoted so much to living-room and dinner-table objets, but the curators have let the extravagance of the pieces tell their owner's story. You inspect at a succession of gorgeous ebony cabinets, armchairs carved with coats of arms, lustrous teapots. Strange things – a clock in the shape of an urn, the numbers running round the perimeter – give way to oriental designs, full of serious Islamic geometrics, and exotic harem impedimenta.

Look at this lovely hookah, in cream-coloured nephrite, its long slender neck set with opals and citrines and dangling with blue stones, its bulbous bowl pregnant with oriental naughtiness, its erect spout lined with silver-gilt. It's a fantasy fetish- object for the Arabist, the furtive harem-lurker, the languid English homosexual with a fortune to spend on self-indulgence. It's a perfect emblem of aristocratic corruption, somewhere between opulence and decadence.

Throughout the exhibition, you're aware of an atmosphere, not of amorality, but of obsession with created beauty. You walk among the vases and ladles and jugs and teapots, the Paul Storr basin – a gleaming cobalt porcelain plate transformed, by its droopy ring-handles, into a miniaturised Roman bath – and its commodes and ravishing cabinets in ebony with lacquer, gilt-bronze, marble and gemstones. You admire the 1600 rock-crystal cabinet, a tiny windowed mausoleum, a box meant for baby clothes made from silver thread and elaborate embroidery. You begin to think: who was this guy?

He was descended from British slave-owning plebeian stock and Scots aristocracy. The family fortune came from Jamaican sugar plantations, established by William's great-grandfather, who was governor of the island. Succeeding generations of vivid roughnecks and uncouth brawlers produced, in William, a spoilt, sensitive, rather feminine boy who was understandably keener on his mother's posh Hamilton forebears (she was granddaughter of the Earl of Abercorn). Beckford's father, a ruthless and philandering politician known as "the Alderman" was twice Mayor of London and died suddenly when William was only nine.

The child inherited an annual income of £70,000 from the sugar estates, a lump sum of £1.5m – and the huge Palladian pile, known as Fonthill Splendens, that his father built in 1755 when his first home burned down. Its wild rococo extravagance, all balustrades and cupolas, pavilions and colonnades and marble statuary, had a profound effect on the bereaved kid, especially the 85ft stone-pillared Egyptian Hall, and the tiny, harem-like Turkish Room, festooned with Persian rugs, Ottoman sofas and painted arabesques. It left Beckford with a passion for oriental splendour and a head full of Gothic fantasies. A further influence was his drawing-teacher. Alexander Cozens, born in Russia, told his young charge stories about the wonders of St Petersburg, and introduced him to The Arabian Nights and to occultism and magic. Impressed by all this counter-Enlightenment exoticism, Beckford began to write. At 17, he wrote The Vision ("a mere ramble through a spectral panorama of peaks, cliffs, lakes and caverns in some indeterminate Orient" sniffed the Times Literary Supplement). At 19, he was in the Lake District, writing emotionally charged descriptions of the sublimity of the landscape, some 20 years before Wordsworth and Coleridge published Lyrical Ballads. And he wrote Vathek, a classic of Gothic literature, the tale of a Caliph whose disapproving eye can kill men, and who sacrifices 50 children and sets off to Istakar to become a servant of the Devil. Vathek is a thinly disguised portrait of Alderman Beckford, while the vast subterranean Halls of Eblis (ie Hell) are spookily reminiscent of the long front hall of his family home.

He was scarcely a literary genius, but a case could be made for Beckford as the first superstar of Romanticism, the first connoisseur of alfresco sublimity, the poet of empty landscape, the pre-Wildean swooner over porcelain and ivory. His own romantic nature got him into big trouble, for he was a bisexual paedophile. On a tour of English country houses in 1779, he met William ("Kitty") Courtenay, the 11-year-old son of his host at Powderham Castle, and fell hopelessly in love with him. His pursuit of the under-age heir was complicated by the fact that he was having an affair with Louisa Beckford, the wife of his second cousin Peter. Word of these entanglements spread through the drawing-rooms of fashionable London in 1780, and Beckford gave the gossips a lot to chew on. His 21st birthday lasted three days, attended by fireworks, bonfires and three Italian opera stars; it was followed by a more decadent Bacchanal at Christmas, masterminded by Philippe Jacques de Loutherbourg, a special-effects maestro who specialised in moving pictures and son et lumière artificial thunder and lightning. A "realm of Faery" orgy of music, feasting, acting and sex ensued for several days and nights. It caused a scandal. Public disapproval forced Beckford into retreat.

He retreated to Fonthill Abbey, the spectacular Gothic church he had built from scratch in 1796. It was supposed to be a large folly, to enhance the park at Fonthill Splendens. But as he worked with the architect James Wyatt, their plans grew more elaborate. The Abbey was based on the 14th century monastery to Our Lady of the Victory at Batalha in Portugal, which Beckford saw while visiting Lisbon two years earlier; he rhapsodised about its "little turrets, flying buttresses, pinnacles and Gothic loopholes". By the end of the year, it was 200ft long and growing. It was ludicrously unstable – the great centre tower collapsed twice during construction – but Beckford didn't care. It was his masterpiece, his waking dream, his vision reified in brick, stone and marble. He gradually pulled down his family home and moved into the Abbey to live, like Kubla Khan, in his stately pleasure dome, surrounded by gorgeous things. But his collecting sprees affected his finances; the falling price of sugar in world markets affected his Caribbean revenues, and he began to run out of money. In 1823, he sold the Abbey to a gunpowder millionaire called Farquhar, and announced a public auction of the contents. It went on for 10 days, attended by thousands. Half the county, and a quantity of titled Londoners, made the pilgrimage to Fonthill to poke around the living quarters of the country's most notorious millionaire.

Beckford wasn't heartbroken by the loss. "I am rid of the Holy Sepulchre," he wrote to a friend. "For 20 years I have not found myself so rich, so independent and so tranquil." On the proceeds he moved to Bath, and bought three houses linked by an enclosed bridge, and built Landsdown Tower, a severe and classical monument to his chilly genius. He died 20 years later, at 84, after catching influenza from walking in an east wind, and was buried in a huge granite sarcophagus designed by himself.

Beckford's was an extraordinary life of extraordinary passions – collecting, travelling, writing, designing, decorating, gardening, acquiring beautiful things, composing and playing music, and falling in love with boys and married women. He was a kind of renaissance man of Romanticism. If you want to see his monument, I'm afraid you can't – Fonthill Abbey collapsed in 1825. But you can feel something of his spirit in the caskets and tables and ebony cabinets at this exhibition – part of the jewelled and lacquered paraphernalia with which he surrounded himself, like somebody gathering the exotic cloths of his childhood dreams around him, to shield him from the disapproving glare of the outside world.

'William Beckford 1760-1844: An Eye for the Magnificent' is at the Dulwich Picture Gallery, London (020-8693 5254) to 14 April