Seventy years later, Sir Denis's keen eye and passion for the Italian 17th century have transformed both the reputation of Guercino and the value of his substantial collection of the artist's works.
And to celebrate his 95th birthday, the historian has curated an exhibition of some of his finest Guercino drawings, along with other important loans, which opens at the Dulwich Picture Gallery in south London tomorrow. "He's one of the great draughtsmen of the Italian 17th century," Sir Denis said yesterday.
When the gallery was founded two centuries ago, Giovanni Barbieri, nicknamed Guercino, was the height of fashion, which is why two great paintings are in its collection. But the artist's reputation sank in the 19th century, not helped by the criticisms of the cultural commentator John Ruskin, who said: "There is no entirely sincere or great art in the 17th century."
That reputation had not recovered by the time Sir Denis, a member of the Guinness Mahon banking family, fell in love with the period in 1933. "Those 17th-century pictures were despised in the 1930s and one had to extract them from that position, which I did," he said. "I thought I would do what I could to resuscitate this artist. But it has taken a long time."
He has been so successful that Guercino's home town of Cento in Italy regards him as an honorary citizen and the price of Guercinos has soared so high that Sir Denis was forced to give up collecting.
The first painting he bought, in 1934, Jacob Blessing the Sons of Joseph, is now worth an estimated £2.8m and hangs in the National Gallery of Ireland. His collection of 75 paintings plus drawings cost him a total of £50,000 yet it is now worth up to £30m - one reason why he no longer has them at home but has distributed them between a number of museums and galleries.
Many of the 44 sheets of drawings which Sir Denis deposited at the Ashmolean Museum in Oxford 20 years ago, and which form the core of the Dulwich show, are themselves worth a fortune. They are second in importance only to the Queen's collection.
Sir Denis's passion for scholarship remains undiminished. After celebrating his 95th birthday next Tuesday, he will resume work on preparing a Caravaggio exhibition that will take place in Italy next year.
Ian Dejardin, director of the Dulwich Picture Gallery, said Sir Denis was astonishing and unique. "He's not a connoisseur who struck lucky. His methods are analytical. He reinvented Guercino from scratch."
What the exhibition shows is the variety in Guercino's drawings, using red chalk, wash, black chalk and ink to depict subjects which ranged from the humorous to the religious. "His pen line crackles with electricity," Mr Dejardin said.Reuse content