Is Harry Blain Britart's most powerful man?

Once upon time there was a great institution, tucked in just behind London's Royal Academy, called The Museum of Mankind. For 30 years it showed off the British Museum's extensive collective of ethnographical objects. About 10 years ago it closed its doors, and the building's been looking pretty woebegone ever since then, used for just a few days once a year to house the Zoo Art Fair. Otherwise, it has been empty and in serious need of refurbishment. Now a man called Harry Blain has brought it to life by leasing it from the Royal Academy and turning it into a private gallery. Except that it doesn't look and feel like a private gallery. It has all the panache of a museum space.

Blain's the name. Harry Blain. But who is this man? Harry Blain runs a gallery called Haunch of Venison, which until this week operated mainly out of a historic building in Mayfair's Haunch of Venison Yard, a former home of Lord Nelson. A former stockbroker who was born in Australia and grew up in Surrey, Blain went into dealing in 1992. He operated out of a first-floor mews space at first, and then opened a gallery in Bruton Street.

In 2002 he went into partnership with a former Christie's dealer called Graham Southern. Since then it's been growth, and more growth. After London came galleries in Zurich, Berlin and, since last September, New York, in the Rockefeller Center on the Avenue of the Americas.

With growth came its natural bedfellow, controversy. Haunch of Venison got sold, lock, stock and barrel, to Christie's in 2007. Is it right for a dealer to be in bed with an auction house? Whatever the ethics, the fact is that the billionaire French industrialist François Pinault, chairman of Christie's, may be the real power behind the throne. Is the strategy deployed over at the Museum of Mankind Pinault's, at least in part?

This is what Blain has been asking himself: in these tough times, how do you lure the collectors to Haunch of Venison and away from, say, White Cube in Mason's Yard, or Larry Gagosian's warehouse-size space in Britannia Street, or Michael Hue-Willams' sassy Albion Gallery in that handsome Norman Foster building just beside Chelsea Bridge on the south side of the river? All these are just as big and brash as Haunch of Venison, and stuffed full of big-name artists too. So what else do you need?

You make sure that the new work you are showing is in a historic building, with some fine detailing to remind you of its venerable history. Pinault did this when he acquired the 18th century Palazzo Grassi in Venice, to show off his own collection of contemporary art. It feels as if Blain is doing much the same here.

A historic building, especially one still very much associated in the mind with a great museum, makes the collector begin to think not only that the work may be of museum quality, but that he himself could perhaps display it in a similar way in his own more modest home. Call it added gravitas if you like. You simply don't get this when the walls are as seamless white, and the floors polished concrete.

The dealer is leading the collector by the hand. I saw it yesterday, in action. Harry was leading them round. Some of them looked pretty awe-struck. The staging is brilliantly persuasive – the lighting; the use of space; the careful deployment of intellectually high-toned wall texts by the likes of Sir Thomas Browne, Walter Benjamin, Italo Calvino. The collector begins to think that he's a bit of an intellectual too, that he's buying into intellectual seriousness. The show comes with a hefty catalogue, in which parallels are repeatedly drawn between ethnographical displays of the kind that you used to see here, and what you can ogle here now. So contemporary art links hands with priceless objects from the past, and gets a corresponding lift in value, seriousness and credibility.

I ask Harry whether all the work is for sale. Practically everything, he tells me. Of the hundred or so, all but four. And what percentage of these artists do you represent? Between 30 and 40 per cent. The global turnover was "several hundred million" this year, he lets out, under a certain amount of pressure. He was sorry that he couldn't be more specific than that, but one has to be discrete about such things.

The big question that hangs in the air is this: is Blain, by this master stroke, now the most important dealer on the block? And what exactly would that mean anyway? All dealers are notoriously cagey about talking cash and collectors, and Blain is no exception to this rule. Consider these factors though. By occupying this institution, he raises his profile and his own credibility hugely. What is more, he has done it in partnership with its owners, the Royal Academy, so he gains by a little reflected glory. The Royal Academy may even snatch the building back in three years' time – that's how long his current lease runs. On the other hand, they may not. It rather depends, at that point, how attractive the partnership is looking to them, and whether Blain is then in a position to make it an attractive proposition that he should stay on because he is, in some way, helping to raise their profile.

All imponderables of course, and light years away. But Blain has in some respects done better than all the others, Hue-Williams, Jopling and even Saatchi. The Royal Academy helped to pay for this re-furbishment, so the cost to him was less than £500,000 – which is little more than the cost of the staging of a major show. What is more, Saatchi's new space in Chelsea, though handsome, doesn't profit by its age – the building inside has been gutted. Only when we stand outside and look back do we really recognize that it was once a splendid 19th century pile.

So Blain has been lucky – but he has one other advantage too: he is personable. This may be an odd one to mention, but it is undeniably true. He doesn't seek out media attention for his private affairs. He has none of that odious, old Etonian swagger. He is surprisingly humble and even congenial to talk to – which comes as something of a shock. He doesn't try to fob you off with half truths. When he doesn't want to tell you something, you feel his evident embarrassment.

Maybe a decent man has the capacity to make it all the way through this world of intrigue and nastiness. Let's not wax too lyrical though. There's still Pinault in the back room, watching his every move on the screens. And Pinault buys hugely for his own person collection from Haunch of Venison, which is an entirely independent subsidiary of course, hem hem ...