The fine art of building up the Tate

However long you have lived in London, or think that you have lived in London, you never get to know more than the few bits relevant to you. Me, for instance - I can never see the point of Fulham. I get as far as something called the Fulham Road ... and then I lose interest.

Of the South or Left Bank I know only a short stretch of concrete running between County Hall and the LWT building. Anything else I would think of as remote, and I suppose that, when I read about the Tate's project of converting the Bankside Power Station for its new gallery, I had a subliminal thought that, however fine the idea, this would turn out to be one of those things that one can never confidently find - like the Barbican, or the Imperial War Museum.

Last week I was invited to look at the site, and one of the first things that hits you is the brilliance of the location. If you stand on the roof of the Bankside Power Station, looking across the river to St Paul's, you can see exactly how you'll get tothe new Tate.

You'll stroll to St Paul's, walk halfway along the south side of the cathedral, hang a right, saunter down to the river and there will be a spanking new pedestrian bridge leading from St Paul's Steps to this weird and wonderful building. It's just a hop , skip and a jump away.

Now Prince Charles is apparently a little worried that this spanking new piss-elegant pedestrian bridge will block the view of the cathedral that river passengers enjoy at present. However, there are ways around this. One would be to settle Prince Charles's hash. Alternatively, the bridge can be set at an angle, so that it does not block the view.

I understand that it is the siting of the power station in relation to the cathedral opposite that has fired the imaginations of the architects on the shortlist for the conversion. Their works will be exhibited only after the winner has been chosen in February.

They must all have stood, as we stood last week, and gasped at the comprehensive view of the City that the roof of the power station affords.

Historically, the area has a pungent character. It was once the brothel district of London, the location of the "stews". Peering down over the power station parapet one sees the almost completed Globe Theatre, which was set outside city limits in rather the way that actors were buried outside city limits. It wasn't respectable. Someone told me (it seems fantastic but is no doubt true) that the decline of this part of Southwark dates from the Puritan crackdown on entertainments.

Between the power station and the Globe there is a house in which it is said that Christopher Wren lived while rebuilding London after the Great Fire. One finds oneself thinking about people like Wren, and about the London of the Blitz and the Second World War, and about all the terrible decisions that have been made since.

One of the visitors with me pointed out a particularly frightful high-rise building. The Tate man said: "Oh, but that'll come down soon, I'm sure. And of course the Barbican will go ..."

I said: "Excuse me, did you say the Barbican will go?"

Tate man, affably: "Oh yes, I think so, eventually. Don't you?" He sounded as if there was nothing at all novel in the idea.

Bankside is huge. The Tate says that the new gallery will be the same size as the Pompidou Centre in Paris, but it seems to be much bigger. The machinery is still in place, as is the electrical substation that serves the whole of south London and will remain where it is, as part of the building.

What will be created is enough space to house the collection of international modern art, to display not all but a good proportion of it, and to keep these displays going for longer than in the past. What remains at the old Tate will be the collection ofBritish art (some items would fit in either collection).

So the old Tate reverts to its original function, while the new one becomes immediately one of the leading modern art galleries in the world. That idea, attractive enough on the printed page, becomes quite giddy-making when you contemplate it on the roofof Bankside. It seems like "one of those things that they do so well in Paris but we never had the brains to do here".

There was talk too of promoting a greater co-ordination between the leading national collections. It would be possible, for instance, for the British Museum to mount its own exhibitions at the new Tate. Supposing there were to be a show of 20th-century German art: it is the British Museum that holds the works on paper. Why should the museum not put on its own show in the new building?

It would apparently be very difficult to reorganise the great collections in terms of ownership without running into serious legal difficulties. What belongs to the British Museum will probably always belong to the British Museum, the same with the Victoria & Albert and the same with the Tate.

But there are plenty of cases where the actual location of objects is simply a matter of historical accident.

The British Museum holds the national collection of icons, but they could equally well be at the Victoria & Albert. Or again, sculpture stops at a certain period in the Victoria & Albert, and picks up in the latter part of the 19th century at the Tate. But these are arbitrary arrangements.

The British Museum's exhibition of Treasures of Byzantium (of which I've so far only flicked through a catalogue) is, in essence, a Byzantine museum created out of 250 items from the nation's collections. This museum will be dismantled at the end of April, but one could easily imagine a decision being made that the nation should have a permanent Byzantine museum, and this could be achieved without any of the relevant institutions losing their Byzantine collections. These institutions would merely becomepart-owners of the new museum.

More temptingly, a national sculpture gallery could - if the wish were there - be conjured out of the joint holdings of the Tate and the Victoria & Albert.

I'm not saying that it should happen, only that it could very easily be made to happen. The amoeba-like behaviour of the Tate is enough to start one dreaming.

In the future, the Tate people hope that you will be able to access their collection, call up a painting by Bacon and find out whether it is (a) at Bankside being international or (b) at Millbank being British or (c) in Liverpool or Cornwall or any art curatorship. Giddy-making stuff.