In 1993, Mr Duthy decided to sell the painting, which his father bought in 1956 and bequeathed to him on his death. The grand and striking picture had blended perfectly with his parents' Tudor mansion, but was too large for his own more modest home.
He contacted Christie's auction house in London, which was a natural enough step. Little did he realise that he was setting in train a remarkable sequence of events - a tale of one painting, two artists and three vastly different prices.
Rachel Mauro, the first Old Masters expert who viewed the work, said that it was almost certainly by Melchior d'Hondecoeter, a 17th century Dutch artist known as "the Raphael of bird painters". She said it would fetch pounds 40-60,000.
Three others - Gregory Martin, head of the Old Masters section at Christie's, and two Dutch consultants, Sam Segal and Fred Meijer - disagreed. They thought it was probably by Pieter Casteels III, a lesser-known Flemish artist of the 18th century, and worth pounds 20-30.000.
The sale went ahead with the picture "attributed to" Casteels, meaning the auctioneers were not entirely sure of its provenance. It was bought by Richard Green, a major London dealer, for pounds 28,000.
And that might have been that, had it not been for the fact that Mr Duthy is no ordinary punter. He runs a company that charts art market sales and even works for Christie's as a consultant.
Flicking through a Christie's catalogue two years later, he was taken aback to come across a familiar work of art.
"There, to my amazement, was the self-same painting for sale, with an estimated value of pounds 40-60,000 and the full attribution," Mr Duthy said yesterday. "In other words, they were saying that they were now certain that it was by Casteels."
The same four experts had revised their opinion of his heirloom, which was bought by a Belgian collector, Leon Seynave, for pounds 48,000.
"I was upset, very annoyed," said Mr Duthy.
Last May, his heirloom came under the Christie's hammer once again. This time it was sold in New York, and this time it was attributed to the "circle of Melchior d'Hondecoeter". It fetched pounds 89,000.
The auction house declined to discuss Mr Duthy's case yesterday.
But experts and dealers say it illustrates the pitfalls facing would- be collectors - particularly in the Old Masters market, where a huge proportion of works are unsigned and undocumented.
Alex Bell, head of Old Masters at Sotheby's in London, said it could be difficult to be certain of the authorship of a work. He said that the circumstances in which a picture is seen can make a considerable difference.
"We're trained to think about these things, of course. But the attribution on a painting with an explanatory notice in a museum is probably believed. In a dirty attic covered in decades of grime and bad varnish, it may be harder to spot the masterpiece.
"A lot of the best-known artists - like Titian, Rembrandt and Van Dyck - are the biggest challenges," he said.
The nature of art scholarship means that there are often experts on lesser known artists where it is feasible to produce a monograph in perhaps four years of a PhD. But for big names the auction houses cannot turn to a single person. Even then, the experts may be abroad and dependent on photographs to try to decide.
Technology has helped. It is known when certain pigments were first created and used, such as Prussian blue in about 1710-1712. "If there's Prussian blue in a work supposed to be by Van Dyck who died 60 years earlier then it can't be Van Dyck," Mr Bell said.
Rembrandt has been well served by an extensive project in Holland which used ultra-violet light and X-rays, counted the strands on a canvas and examined the wood in his known works. One of the consequences, however, is that there are now thought to be perhaps 350 Rembrandts in the world compared with up to 1,000 at the beginning of the century.
Alex Bell believes it is possible to get the problem out of perspective. Italians rarely signed their works and as much as four-fifths of the paintings produced between 1300 and 1800 may have no signature. But from the 17th century, Flemish and Dutch artists increasingly did.
t A painting was sold at a country house sale in 1995 for pounds 155,000. After extensive renovation it was revealed as The Sack of Jerusalem, a 17th-century work by Nicolas Poussin, worth pounds 8m to pounds 12m.
t Venice, View of the Grand Canal with Santa Maria della Salute was thought to be by a follower of Luca Carlevarijs. Only when it was hanging for sale did Sotheby's identify it as by Carlevarijs himself. Six months later it made 20 times its previous estimate.
t A work thought to be "just a French 17th-century painting" was revealed as an early landscape by Poussin worth pounds 2.35m, which had been in the collection of the 18th-century British master Sir Joshua Reynolds.
t A Boy Drinking was sold for pounds 209 in Somerset in 1984. A London dealer, Derek Johns, bought it for pounds 5,500 and then sold it for nearly pounds 1.5m, by which point it had been identified as by Annibale Carracci.
LOUISE JURYReuse content