Artworks whose real value has been missed are called 'sleepers'. Someon e's going to make a killing...
Parked outside Christie's, Sotheby's and the big dealers in Bond Street, you can see dusty Volvo hatchbacks from which piles of grubby oil paintings are being unloaded by men in tweed jackets and jeans. They are "runners", the hyenas of the art-market. Bleary-eyed, they have been on the road from the country since dawn. The paintings have been snapped up from country auctions in the hope that they are "sleepers" - artworks whose real value has been missed by the auctioneer and rival bidders.

Now the runners are in for the kill. Among their finds might be a heavily varnished picture of horses, modestly catalogued as "18th century English school" at a country auction. The runner has laid out pounds 1,000 on it and is about to try to convince the expert at Christie's front counter that it is a Stubbs worth anything up to pounds 500,000. The runner's unwitting allies: inexperienced small auctioneers such as the Isle of Wight firm, now defunct, which catalogued a Stubbs as by "S Tubbs".

The most renowned runner is Philip "Buffy" Parker, closely related to the Earl of Macclesfield, whose eye for art transcends that of any hyena. News of his arrival at front counters brings a Pavlovian response from auctioneers. His other nickname is "the Hoover". He will Hoover up 40 canvases a week, having driven 1,000 miles and viewed 5,000 pictures at 20 country auction houses. His hits have aroused such jealousy among rivals that he now employs decoys to bid on his behalf. Otherwise he becomes the victim of "bitch bidding" by local traders - they try to "run him up", bidding higher and higher, then dropping out, lumbering him with a higher price.

Buffy's haul of thousands of paintings is stored in the grand country house in Hampshire that he bought with the proceeds of his biggest hit of all - a grimy portrait of a Pope, painted on slate rather than canvas, for which he laid out a paltry pounds 180 at Sotheby's one-time saleroom in Chester. Buffy sold it for pounds 380,000 at Christie's, London. It was the Venetian High Renaissance master Sebastiano del Piombo's portrait of Pope Clement VII. The Getty Foundation in America eventually bought it - for a reported pounds 6.5m. Sotheby's found themselves with some out-of-court settling to do.

But such is the romance of art-market finds that the ever-opportunist London auctioneers have taken to publicising them as crowd-pullers - not the "sleepers" that have slipped through their fingers, of course, but the clever discoveries they themselves have made before the goods have come under the hammer. Nowadays, any lot not fresh to market is likely to be given the "discovery" treatment. For example, Sotheby's describes as "newly discovered" two watercolours by the great Victorian bird painter John Audubon, estimated to fetch up to pounds 70,000 on 11 July. Newly discovered? Were they lost under a sofa or buried in mouse nests? No, simply stored in a private collection.

It is not just bleary-eyed runners who line their pockets by awakening sleepers. All dealers earn their living by finding greater value in artworks than was previously perceived.

At auction, you can tell the paintings that have undergone examination by dealers. They appear at the rostrum covered with cloudy smears like a dirty car windscreen. The smears are spit. Dealers in search of sleepers have been licking their fingers and dabbing at the grubby surface, creating portholes through which the colours of the paint shine through, like a dry pebble washed by the tide. The sideways look, the darting spit-and- dab, are all part of the battle of wits played for high stakes between dealers and auctioneers.

The auctioneers have yet more tricks up their sleeves. An insidious new invention of theirs is the "sleeper on a plate". At Sotheby's sale of contents of the Marquess of Bristol's East Wing at Ickworth, Suffolk, this month, were some 60 portraits, dumped there by the National Trust when they took over the rest of the stately home. They were catalogued tantalisingly as "studio of", "school of", "follower of" Van Dyck et al. The auction was packed with international fortune-hunters. The "sleeper on a plate" was lot 484, a portrait of the Marchesa Balbi. Although the catalogue entry said that Dr Susan Barnes, the acknowledged authority on Van Dyck, had endorsed attribution to the master, and would be including the portrait in the forthcoming catalogue raisonne of his works, Sotheby's headlined it as "attributed to" Van Dyck - a coded tag meaning that the auctioneer considers the attribution no more than "probable". Moreover, the estimate for the Marchesa was a ludicrously small pounds 4,000-pounds 6,000, strengthening the misleading impression that Sotheby's, against all evidence, considered the attribution to Van Dyck little better than worthless.

Result: the painting was well and truly spat upon in the time-honoured manner and bidding, in the absence of a sensible top estimate, went through the roof. The painting sold for pounds 133,500. Normally, an "attributed to" estimate on such a Van Dyck would be in the region of pounds 20,000-pounds 30,000, an attribution to the master's hand between pounds 40,000-pounds 60,000 and pounds 60,000- pounds 80,000. Such is Sotheby's riposte to sleeper seekers.

There was a Pope in the sale, Alexander VII, lot 475, catalogued as "Roman School, 17th century". It sold for pounds 10,350. As he brought the hammer down, the auctioneer, James Miller, announced, a little truculently, the buyer's name: Philip Mould. I heard a gasp all around me. Philip Mould is Britain's sleeper-finder in chief. Aged 36, he has bought for a song countless sleepers in his field of expertise, British portraits, and is author of a bestselling book, "Sleepers". It is unusual for a buyer's name to be sung out at auction. It was as if Mr Miller was telling bidders: "There, see what you've missed!"

Mr Mould has a donnish, book-lined gallery in Bond Street that exudes opulence. Propped nonchalently against the wall is his latest find, a portrait in oils of King Charles II by the 17th century Italian Antonio Verrio that used to be the centrepiece of the ceiling in St George's Hall, Windsor Castle. The trade expects the Queen to buy it but Mr Mould is saying nothing.

The portrait was mis-catalogued "Kaiser Leopold I" in the Dorotheum auction rooms in Vienna. He bought it for a mere pounds 8,000. The portrait was thought to have been destroyed on the orders of George IV, who was jealous of his Stuart ancestors.

Mr Mould knows the heart-thumping excitement of bidding without knowing whether other dealers in the room have spotted the same sleeper, the silent prayers offered by the fortune hunter for the hammer to come down. Then there is the elated apprehension as the skilled restorer dabs gingerly at later-added paint with a cotton wool swab soaked in acetone.

Propped on an easel in his gallery is another celebrated find of his, worth pounds 500,000 - a tiny portrait by an unknown artist of Prince Arthur, elder brother of Henry VIII, who died aged 15 in 1502 before he could ascend the throne. It was correctly catalogued by Sotheby's, but only Mr Mould and his researcher twigged that it was the only original portrait in existence of the frail-looking Prince, whose untimely death changed the course of British history. It was estimated at pounds 4,000-pounds 6,000 and he bid pounds 12,000 for it.

Mr Mould's motto: "Fortune favours a prepared mind". He says: "Knowledge of an artist's style is paramount, but it is not just cerebral: it is intuitive, almost sensual. You need to become familiar with each artist's voice, even if it is disguised or speaking with a foreign accent - for example, English 18th century painters were influenced by French and Italian styles".

He is disdainful of some lofty art historians, especially "acknowledged authorities" - those museum curators whose God-like pronouncements can make or break an attribution. "Some art historians", he says, "lack the connoisseurship to identify traits of the artist for whom they're meant to be the authority. Unlike dealers, the expertise of some of them often derives too much from photographs.

"We see, say, 50 pictures a day and develop a skill that some of them sadly lack. Also, the financial risks we take do have the effect of sending oxygen to the brain". So why are there no dealers who are acknowledged authorities? Simple: "Because we are financially motivated".

"Sleepers: In Search of Lost Masters", by Philip Mould, Fourth Estate pounds 16.99.

Bought for a song, sold for a fortune...

Discoveries: hooray for the auctioneers.

Assyrian carved stone bas-relief, 3,000 years old, discovered in the tuckshop of Canford School, Dorset. Sold for pounds 7,701,500, Christie's 1994.

Dancing faun bronze, 31in high, by the Renaissance sculptor de Vries, rescued from Sotheby's garden sale and an estimate of pounds 1,200-pounds 1,800 by an alert employee, auctioned for a record pounds 6.82m in 1989.

Discovered in a private collection in South Africa as a result of a trade "probe" by Christie's: two unrecorded Canaletto paintings that fetched pounds 388,800 and pounds 354,600 in 1985.

Cache of Durer prints found in a trunk among nesting mice fetched pounds 587,952 at Sotheby's two years ago.

Schoolchildren studying Egyptian hieroglyphics persuaded their teacher to call in Sotheby's to identify a carved stone in the garden - it was 12th Dynasty Egyptian and fetched pounds 117,100 two years ago.

Handwritten page from George Washington's inaugural speech as president of the United States, found under a sofa in Suffolk. Sold for pounds 180,000 this month at Philips.

Moss-covered marble statue of cupid with bow by the Italian master Canova, expected to fetch over pounds 1m at Sotheby's, July 4.

This month, after being lost for 60 years, the painting La Danse Pyrrique by French Orientalist Leon Gerome, sold for pounds 804,500 at Christie's.

Unnoticed amid the clutter of a disused scullery in the Midlands, a hoard of Egyptian antiquities is expected to fetch pounds 20,000 at Christie's on 3 July.

Sleepers: hooray for the dealers

Marble Venus by the Florentine Jan Bologna, bought for a sensational pounds 715,000 against pounds 3,000-pounds 4,000 estimate by the London dealer Alex Wengraf at a Christie's garden sale in 1989, thought to be worth pounds 6m.

Renaissance painting of Pope Clement VII by Sebastiano del Piombo, bought for pounds 180 by Philip Parker at Sotheby's Chester, sold Christie's for pounds 380,000, 1988.

Portrait of Prince Arthur, heir to the Tudor throne, bought for pounds 12,000 by Philip Mould at Sotheby's three years ago: worth pounds 500,000.

Bought at auction by Philip Mould for pounds 9,350 five years ago, a "circle of" Van Dyck portrait said to be of Countess Carnarvon, but revealing after removal of overpaint Countess Dysart by the master's hand.

Original Van Dyck portrait of a couple, obscured by dirty varnish, bought for pounds 75,000 at Sotheby's, New York by Richard Knight of Colnaghi and sold for pounds 450,000.

Poussin's 1626 Sack of Rome, catalogued as Testa's Sack of Carthage by Sotheby's last October, bought for pounds 155,000 by London dealers Hazlitt, Gooen and Fox: thought to be worth up to pounds 10m.