When curators at the British Museum were approached in 2005 by an elderly man with a broad Lancashire accent asking for their opinion on a fragment of an Assyrian stone frieze he said had been in his family since 1892, they had every reason to believe he was just the latest genuine enthusiast to seek their expertise.
Only when George Greenhalgh, an 84-year-old former technical drawing instructor from Bolton, hinted that the family would be willing to part with their prized artefact for £500,000 did the experts' concerns about the authenticity of 2700-year-old art turn into full-blown suspicion.
On closer inspection, they noticed that the carving of a bearded horseman leading two steeds, supposedly part of a documented bas relief presented to the Assyrian monarch Sennacherib for his "palace without equal" in what is now Iraq, showed atypical harnesses and, crucially, a spelling mistake in the ancient Mesopotamian script. The frieze was, in fact, an extremely clever fake.
The museum contacted Scotland Yard's art and antiques unit, which began an 18-month investigation into Greenhalgh, his 83-year-old wife, Olive, and their 47-year-old son Shaun, an antiques dealer.
Its detectives were astonished by what they found at the Greenhalghs' home. Their council house contained a collection of phoney treasures, each painstakingly forged in original materials from Egyptian glass to Roman silver, and detailed "histories" culled from obscure archaeological records and historical texts.
From a monumental Roman silver tray supposedly dug up in Derbyshire in the 1720s to paintings by L. S. Lowry to a sculpture by Barbara Hepworth, the family were responsible for a cottage industry producing at least 120 forged art works which, if they had all been sold at market rates, would have been worth £10 million ($27 million).
Some items were so perfectly executed by Shaun Greenhalgh that they fooled experts at leading auction houses and museums.
Shaun, the mastermind of the fraud, was sentenced on Saturday to four years and eight months in prison, and his mother was given a suspended 12-month jail term after they admitted conspiring to defraud museums and private collectors over 18 years.
George Greenhalgh, who is wheelchair-bound and suffers ill health, has pleaded guilty to conspiracy and will be sentenced after a medical report.
Detectives said the family were responsible for the "biggest and most diverse" production line of faked artworks the Yard unit had ever come across.
Detective Constable Halina Racki said: "What is remarkable is the sheer breadth of what was produced. They went to great trouble to source the correct stone for faking Egyptian statues, and bought Roman silver to melt down into the Roman tray. Some of it is extremely well executed."