But this is not an art gallery: it is a speculative office block. Such buildings usually don't offer architects much scope since they are just construction kits where the architect's aesthetic input is confined to the outside skin and the entrance lobbies. The interiors are left as blank spaces and are usually fitted-out by others. This building is, as you would hope, different. Although it has many such blank spaces awaiting tenants - redeemed by the curious shape of the floor plans- there is much else as well. For instance it is possible to walk right through the centre of the building at street level, or emerge up into it through shops and cafes from the Underground. The central lightwell, clad in deep blue tiles with brightly-coloured window frames overlooking the space, sets up the whole geometrical game of the building, which is to juxtapose circle with triangle. So you get triangular levels within the central circular drum, which itself is within the overall triangular shape of the building. This play of geometries is taken right through to the detail of paving slabs and ceiling tiles.

From way down below, you look up into the drum and see a cowl-like object projecting into the top of the space, like a goods hoist. Take one of the colour-coded lifts from the quasi-Mycenean public lobby to the roof, and you realise that this cowl - a copper-clad object similar to the roof of Stirling's little bookshop in the Venice biennale gardens - serves no purpose whatsoever but to intrigue. It suggests that something is going on up there, and it is: a half-acre rooftop garden in three sections, landscaped by Arabella Lennox-Boyd after Stirling's design. Part of this is spill-out space for a restaurant, the Coq D'Argent. Sir Terence Conran, who came up with this punsterish name (coq=poultry, argent=silver=sterling), expects to have an annual turnover of £6m in this heart of the expense-account lunch belt, and no wonder: there is no place like this in the whole City. Office blocks nearly always waste their roofs by dumping water tanks and airconditioning plant all over them. At Poultry, all that is hidden in the basement and the gardens are the delightful result. From the restaurant area you move to an oak pergola, festooned in wisteria, running round the top of the drum. Then Palumbo opens a gate to take you into the private garden leading to the apex of the building. Here the walls are low. A verdant zig-zag shaped lawn flanked with echelons of low box hedges takes you to the "bridge" - the two timber glass-sided decks projecting either side of the clocktower marking Bank junction. Six floors up feels very high here, and vertigo sets in - but not before you notice that one arm of the bridge leads your eye directly to the tower of Wren's St. Stephen Walbrook, and the other to the mid-point of Lutyens' superb 1924 Midland Bank. Accident or design?

In the lower regions, Stirling indulged the doomy Hawksmoorish side of his character. The so-called "grand staircase" leading from the VIP's entrance beneath the clocktower up to the first floor turns out to be a long sloping stone-arched tunnel with deep, shallow steps, overlooked by little box windows. Palumbo thinks it is like a corridor leading to a Pharaoh's tomb. So it is: then again, I half expected to find a giant egg rolling down it.

Lots of people still cluck over Jim's big fat hen at Number One Poultry. I suspect this is because, deep down, they don't really like the idea of a modern building with all the eccentric character of an earlier age. It is not pretty. It is not graceful. It does not ingratiate itself. It is defiantly strange. But it exudes an astonishing sense of power and purpose. I admire it, but I don't yet like it. And I know the hen doesn't give a damn what I, or anyone else, may think.