William Beckford - aesthete, Gothic novelist and proto-decadent of a decidedly Proustian cast - cut a startling figure in late 18th-century England, even after scandal put paid to his public career. Roger Clarke profiles the man, and surveys an exhibition dedicated to his leavings.

At the age of 21, William Beckford inherited the largest Jamaican sugar fortune ever made. Five years later, in 1786, he embarked upon what promised also to be a glittering literary career with the publication (originally in French) of his oriental Gothic novel, Vathek, the tale of an effeminate caliph who sells his soul to an evil jinn in the hope of assuming the power of the pre-Adamite sultans.

Then scandal struck. Beckford was discovered in the bed of the young and foppish heir to Powderham Castle, William Courtenay. His reputation was destroyed overnight. He became a social outcast, despised and declasse. With his gregarious nature, he could not cope with his sudden demise, and retreated into a fantasy world of grandiose building projects and the bitter and assiduous hoarding of beautiful objects.

The similarities between this man whom Byron called "a martyr to prejudice" and that other fin-de-siecle society victim Oscar Wilde (almost exactly a century on) are striking. Vathek plays a role in Beckford's life very similar to that played by A Picture of Dorian Grey in Oscar's- it was a public love-letter to the very boy who would destroy him. As with Wilde, Beckford's most implacable foe turned out to be the sociopathic father of the boy (or stepfather, in Beckford's case) and, as with Wilde, his fate was sealed by the feckless narcissism of the gilded youth in question.

Wilde was broken by his treatment (far worse, of course, than Beckford could ever have imagined); but Beckford remained defiant till his dying day, building towers, some taller than Salisbury Cathedral, to force people to take notice of him. His vast Gothic "abbey" at Fonthill in Wiltshire, designed by James Wyatt, later inspired a generation of country house design, even though it collapsed during his lifetime (curiously, nearly all the country houses based on it have vanished also). The brightly coloured mock-medieval interiors struck a chord with a generation of neo-medievalists, including Pugin and William Morris. The remains of his vast and valuable library, his collection of incomparable paintings, furniture and china, still continue to eddy round the sale rooms 150 years after his death, like the detritus of some huge island recently sunk beneath the sea.

There is no doubt that Beckford had what Bruce Chatwin called "the eye" when it came to art. For a while, with his fortune behind him, he could outbid princes of the realm. Though his greatest lasting achievement is literary, it is as a collector of that mock-tidy and dubious concept "taste" that Beckford is mostly perceived. And despite a contemporary attack on this idea of his collecting abilities by the essayist William Hazlitt, this is the image of him that has stuck.

Sadly, the current exhibition at Christie's - mounted as a fund-raising exercise by the Lansdown Trust, whose aim is to restore the peculiar tower Beckford built in the garden of his last home in Bath in the 1820s - is a tad disappointing. Lottery money has been secured; attempts have been made to buy back Beckford's furniture when it comes on the market. But the truth is that the Lansdown Tower was built specifically to house a particular, long-vanished art collection, and what colour the carpets were or how the ceiling looked are irrelevant without those objects there. Lansdown Tower without the treasures in it will be a sorry affair, as this exhibition inadvertently reveals this.

Beckford's rather nasty medal cabinet, designed by himself, and his slab- like polished sofas (like most of the display, from the late James Lees- Milne's collection) provide a scanty picture. It is a reminder that Beckford's life at Bath is the least interesting period in his life, when he turned his back on the Gothicism that made him famous and made no further big purchases for his collection.

As with the tower, so with the exhibition. One looks in vain for Raphael's St Catherine or Bellini's serene Doge Loredan (or any of the 20 Beckford paintings now at the National Gallery) which once hung in the tower. There is no grotesque "Blackamoor Stool" from Brodick Castle (a reminder that his money came from exploiting Jamaican slaves). Don't bother searching for the sumptuous Reisener secretaire from the Wallace Collection, on which the disgraced Beckford wrote his letters to London, letters full of scurrilous assignations with rent boys and circus performers, and which he pretended had belonged to King Stanislaus of Poland. Forget the Augsburg Cabinet (fingered perhaps by the French dwarf Perro, who ran his household), now in the V&A, which Beckford fancifully insisted had been at Whitehall Palace. The painted coffers which he claimed had belonged to James I, but which he had probably made himself? Not there. None of the real treasures in Beckford's collection - the Rubens vase, the "Van Dieman" black lacquer box that once belonged to Mme de Pompadour, perhaps the Meissen dinner service of Stadholder William V, the prints of William Blake - are hinted at in the exhibition. There is not a whisper of the silver Beckford secretly bought from George IV when the latter was low on cash.

And what of his collection of books and manuscripts? His original Edward Montagu Arabian Nights manuscript, for example, now housed in the Bodleian, the source material for much of Vathek, would have been a fascinating object to see.

The ladies of Bath who run the Trust unconsciously seek to turn Beckford's final bolthole into a kind of picturesque folly of an eccentric, full of knick-knacks and cinquefoil wallpaper, with Beckford packaged like some linen sachet of National Trust lavender. As a Regency eccentric, he is marketable. As a martyred pederast, he is not.

The ladies of Bath have turned a blind eye to his sexuality. In truth, his time at Lansdown was not all art and decor - there were also the footmen with female names, such as Mme Bion and the under-age pageboy Ali Dru, the secret pornography collection, Beckford's affected singing like an Italian castrato. After all, this man of taste had other tastes, too, less palatable to the heritage tourist industry and respectable charities.

The restoration of the Lansdown Tower is an empty gesture laden to the hilt with good intentions. Better to remember the vanished splendours of Fonthill; better still to revel in the mercurial humour of his only true and lasting work, Vathek.

It is curious that after all these years Beckford is still presented first and foremost as a man of "taste", in the 19th-century sense of something objectively "correct", when the truth is that his taste was fickle and had far more to do with his state of mind than with any statement of what is acceptable.

That he had taste is still used as a callow euphemism for his homosexuality. What would he have thought of Dr Roger Scruton, whose delightful wife Sophie is at the helm of the drive to restore Lansdown? Dr Scruton has written at length on the subject of homosexuals being outlaws who should be excluded from having any stake in society. Were Beckford to stalk forth from his ebeniste furniture, miraculously restored to life, he would find that some things haven't changed at all.

The Beckford Exhibition at Christie's in King Street runs till 3 February. Tonight at 6.30pm Christopher Woodward will give a lecture on `Beckford and Goodridge in Bath'. A new biography of William Beckford is due to be published in June.