This Wednesday, Sept 16, a huge party will animate the Piranesian spaces of Giles Gilbert Scott's old power station at Bankside, looking across to St. Paul's Cathedral. Serota's Swiss architects, Jacques Herzog and Pierre de Meuron, have fulfilled their promise: they are transforming the space into one of the world's great art museums and it is on time and to budget.

True, there is still the little matter of finding a shade under £20m of capital to pay for it, and a further £5m every year to run it thereafter. Plenty of potential donors have party invitations on their desks. Meanwhile another £1.8m remains to be found to finish paying for the huge rebuilding programme of the British Galleries now under way at Millbank, where the new rooms are designed by Serota's long-time collaborators going back to his Whitechapel Gallery days, the architects John Miller and Su Rogers. But the director calmly announces that the money will be found, the gaps will be bridged, and you believe him: these projects have gone far too far to stop and besides, every other lottery-funded project in the country is in exactly the same boat. If he fails, everyone fails.

Now the Tate has commissioned the corporate identity specialists Wolff Olins to come up with ideas as to how to present the impending dual-identity London Tate and to come up with snappier, more memorable, names than the two on offer at present: respectively the Tate Gallery of British Art (Millbank) and the Tate Gallery of Modern Art (Bankside). There is the potential for confusion, no doubt about it. Serota says: "The question that's always asked is - where will I find Francis Bacon, or Damien Hirst? - and the answer is really quite simple: in both. What you will find here at Millbank is 20th century British art in the context of a tradition of British art going back to 1500 - and there at Bankside you'll find 20th century British art in the context of an international 20th century".

All clear, everyone? Right: here's the Serota line on the weaknesses in his international modern collection: "One answer is that museums are only partly about the inherent quality of the collection. As important, in my view, is the way in which they use that collection. So you can have a brilliant collection, but display it rather poorly, and your museum will not flourish." The implication being that the Tate has a rather poor collection compared with some, but will display it brilliantly: however, he does not say this. The remark puts into context his desire for a £10m gift to buy new art for Bankside. "An imaginative gesture of that kind would make a big difference," he muses, while allowing he will probably be told to try the Lottery again, with uncertain success.

The affair of Bacon's studio puts him especially on guard. The Tate was never approached with a formal offer for it, he says. He had sporadic conversations with John Edwards, inheritor of the Bacon estate, but it never came to anything. Did he regret the loss, I ask? He replies at a tangent. "The priority for us has to be showing Bacon's paintings rather than his studio." He then praises the "whole experience" of Brancusi's studio in Paris, or the Tate's Barbara Hepworth studio and garden in St. Ives. So is the Bacon studio not up to that standard, I inquire? Serota sits up and stares straight at me. "Well - we'll see," he replies, with a short laugh.