Except that there's something odd about his walk. Bent with age, he uses a stick. But he doesn't shuffle or meander. Rather he marches. Quick and economical, this is a single-minded march.

The walk is the key to the man. For this is Sir Denis Mahon, who has never been in two minds about anything. Ask anybody who knows him what he's like, and the first thing they'll say is 'single-minded'. I discovered for myself the depths of this single-mindedness. When I asked him questions, he stopped walking to answer them. And at both our lunches, he refused to eat and talk at the same time. 'One thing at a time,' he says, 'one thing at a time.'

Civil servants at the Department for Culture, Media and Sport (DCMS) and the Treasury are afraid of this little old man. Anthony Blunt, the art historian and traitor, quailed before his onslaught. Tony Blair has been seen to panic at the sight of him. Gordon Brown, wisely, did what he was told.

Mahon is an art historian and collector. Single-handedly and single-mindedly, he saved an entire century of Italian art from critical oblivion. In the process, he bought pictures - 79 of them. Mostly he bought them for £100 or £200, sometimes he went as high as £2,000. His whole collection cost him a mere £50,000. Today it is worth anything up to £40m, possibly much more. Now, in preparation for a death he faces with organised equanimity, he is giving it all away - a third of it to the National Gallery in London, some to the National Gallery of Ireland, some will be returned to Bologna, where they were painted, and the rest will be dotted around the country and the world.

But there are strings attached to these gifts. If these galleries ever decide to charge for admission, they will at once lose all their Mahon pictures. The same thing will happen if they attempt to sell any of their permanent collection. He has also used the power of his collection to bludgeon the government into allowing national museums that do not charge an admission fee to recover Vat - a move that has transformed the museum business in London. Years ago, he successfully fought to get works of art accepted by the Treasury in lieu of death duties. So I say to him over lunch at the Cadogan, are you quite happy to use your collection to blackmail people?

'Oh yes,' he chuckles, 'blackmail, of course. What's the point of having a club if you can't wield it? And it's in a good cause. Pro bono publico, damn it. Not democratic? That's tripe. As we all know, a lot happens for good which is not produced by democratically elected people. Do you see?'

All of this can only be understood in the light of two things that happened in the mid-19th century. In 1838 the Guinness Mahon bank was founded, and in 1843 John Ruskin, the great art critic and social visionary, published Modern Painters. The first provided the family wealth that was to underpin Mahon's career. The second provided its aesthetic justification.

Ruskin - 'the nuisance', Mahon calls him - despised the Italian baroque painting of the 17th century - the seicento. He had a puritanical loathing and mistrust of its lushness, religiosity and emotionalism. His views were immensely potent. When the National Gallery bought two paintings by the baroque artist Guido Reni, Ruskin condemned the purchase so fiercely that the gallery did not dare buy another seicento work for over half a century. What Ruskin had done was set down in prose of genius a deep and ancient English prejudice. For that was the art of the counter-Reformation, the Roman Catholic backlash against Protestantism. Its highly coloured religiosity was rampant Catholicism. Later, modernist critics, who also disliked lushness, shared Ruskin's view and, but for the occasional dissenting voice, the conviction spread throughout the art world that seicento art was all but worthless.

Enter Denis Mahon, the only child of parents who had inherited Guinness Mahon money. He had three Irish grandparents and he still holds dual citizenship. He was privileged but solitary; when he was 12, his parents took him to Italy.

'We used to go to art galleries and churches - the usual tourist stuff. On this sort of grand tour, I found that I was rather good at this rather amusing game - the attribution game. I found that I could recognise these painters from some distance away. It just happened, you see -I could always say, 'That picture is by so-and-so.'He had discovered his 'eye'. He went to Eton and then Oxford. But the university had no art history course and he took an ordinary history degree. However, Oxford did have, as the director of the Ashmolean museum, the great Kenneth Clark, later to become a television star with his series Civilisation. Mahon stayed at Oxford an extra year just to sit at his feet.

'He was very helpful, and by that time I couldn't understand, from looking at 17th-century Italian paintings, why they were so despised. They were absolutely despised.' Clark encouraged Mahon to study the period by attending lectures at London's newly founded Courtauld Institute.

There, in 1933, he met another great historian of art, Nikolaus Pevsner, who taught him the methods of art history. He asked Pevsner, who was also one of the few fans of the seicento, what artist he should study. Pevsner suggested Guercino and thus launched one of the most extraordinary careers in British - or, indeed, world - art history.