Yesterday afternoon, with Neil MacGregor, director of the National Gallery at his side, Sir Denis publicly announced his decision - one that was made privately several years ago - to leave 79 works to the nation on his death. But on one condition: if one of the galleries or museums concerned sells off a single work from its public collection, at any time in the future, the bulk of the incomparable Mahon collection will be withdrawn from public view and put into the care of the National Art Collections Fund.
Sir Denis has, he hopes, the Government over a barrel. If it wants - it doesn't, but Neil MacGregor and his colleagues do - to put this unparalleled private collection on permanent view, it must promise to stop selling off the national silver. Sir Denis is 86, and as he watches the Government cut spending on the arts - the National Heritage Memorial Fund which buys works of art on behalf of the nation has just had its budget slashed from pounds 8m to pounds 5m - he feels the time has come to fight on his terms, supporting the public realm by placing his valuable paintings there.
"Denis has been a thorn in the side of governments for years," says his old friend, Sir Brinsley Ford, the grand art collector and two years Mahon's senior. "He has always been on the attack - years ago when a large part of the National Gallery's collection was to have been sold and, again, when many of the minor pictures at Dulwich were to have been flogged off. Now Denis has the most diffident, almost Chinese-like manner of anyone I've ever met, but beneath that polite exterior is one very tough man."
Sir Denis has also been leading a campaign in the letters columns of national newspapers to discourage the introduction of museum charges. He might be high-minded and one of the last of the amateur connoisseurs of the Brinsley Ford-Harold Acton era, but he is no snob and wants anyone and everyone to share in his lifelong passion for the art of the Italian seicento.
And we are talking of a love affair. Sir Denis has lived alone in Cadogan Gardens, surrounded by his magnificent collection, since his mother died in 1970. Few, save students at the Courtauld, fellow connoisseurs, historians, museum and gallery curators have feasted their eyes on the voluptuous and bombastic oils, notably by Guercino, that line the panelled walls of this handsome, if discreet Italianate house.
"I spent an extra year up at Oxford," says Sir Denis, "studying informally at the feet of K Clark [Kenneth Clark, then director of the Ashmolean; later Sir Kenneth Clark, director of the National Gallery and Lord Clark of Civilisation]. K encouraged my nascent interest in the Italian Baroque. This developed further when Nikolaus Pevsner arrived as a refugee from Germany and began to teach at the new Courtauld. Pevsner had been an assistant at the Dresden Gallery and was an expert in the Italian seicento. He was brilliant and broke, I was keen and rich, so I took private lessons from him."
How rich? Sir Denis does not say, but he was a lucky young man who inherited a small fortune from his family connection with the Guinness Mahon merchant bank. An aesthete with a banker's acumen, he bought well from the early Thirties, but without financial gain in mind.
He comes from what he calls the "Irish Raj", that irascible and stubborn breed of Anglo-Irish; his father was John FitzGerald Mahon, his mother Lady Evelyn. He is grandson of the Fifth Marquess of Sligo, and, as the critic Brian Sewell puts it, "a good, old-fashioned connoisseur. Denis knows more about Italian seicento painting than any pompous young popinjay flaunting a doctorate as if this meant they actually cared about art, let alone knew much about it. Denis cared about Guercino and Caravaggio at a time when any old dark, candlelit Italian oil was called a Caravaggio and worth next to nothing. The point is he really looked at paintings and knew their value in every sense; above all he loved their beauty, not a word that one is meant to bandy about today and certainly not since Mrs Thatcher, who thought art galleries some sort of supermarket for pictures.
"In another age, I imagine Denis would have been one of the desert fathers, an ancient and ascetic divine in love with God rather than Guercino."
Sewell's comment is all the more generous because it was the terrier- like Mahon who first dented the reputation of Anthony Blunt, grandest of art historians and Sewell's friend and mentor. In 1960, the first comprehensive exhibition of Poussin's work was shown at the Louvre. Blunt hung the paintings. Mahon pointed out, convincingly, that Blunt's chronology was out of order. Thirteen years before he had published Studies in Seicento Art, described then and now as "seminal": the amateur had rattled the art world's most brilliant professional.
"Denis was always far in advance of the rest of us in his taste and knowledge of old master paintings in the early Thirties," says Brinsley Ford. "I sometimes wonder if it was the frightful asthma attacks that holed him up, particularly in the summer months: he focused his brilliant and undivided attention on his subject. No wonder he was able to revise our opinions and knowledge of Poussin as well as Caravaggio and Guercino."
Italian Baroque had been dismissed by John Ruskin, the great Victorian art critic in his serial tome Modern Painters; it only began to come back into its own in the 1920s with the publication of Sacheverell Sitwell's Southern Baroque Art (1924), an atmospheric if unscholarly celebration of an immensely visceral and operatic form of painting that became increasingly popular amongst aesthetes, not least as a reaction to the rising tide of Modernism. Mahon has yet to buy a work by a contemporary artist.
Of his superlative collection of Italian masters, Mahon says he bought most of them in auction in London and the rest through the Barberini family, who appeared to have enjoyed a unique agreement with Italian governments during and after Mussolini's rule, whereby they could sell works of art abroad. No question of Mahon sending paintings back to Italy then? In fact there is. Although the bulk of Mahon's collection will stay in Britain, seven important works are destined for the Pinacoteca Nazionale, Bologna. Mahon has divided his pictures between galleries and places that have played a key part in his life - there are paintings for the National Gallery of Ireland as well as the National Gallery, and for Oxford's Ashmolean, where he first fell into the embrace of the Italian Baroque.
Will he be sad when the last of his beloved Guercino oils has gone from Cadogan Square to public gaze? "Sad? Oh, terribly. It's been a long and very lovely affair, but it's time to go public."
He will, he says, allow works he has as yet kept at home to go abroad if he cannot persuade the Government to put a brake on the introduction of admission charges to public galleries and museums. And he does have a few choice Seicenti up his immaculately groomed sleeve: he reserves the right to remove paintings from his bequest if he believes the Government is acting to make public art inaccessible n