We met before the bridge debacle, on his birthday, in his vast riverside studio in Battersea. Foster, who likes to fly planes, compete in mountain-ski-ing marathons, and do a lot of running and cycling, is unlikely to take up his free bus pass. His youngest child and only daughter, Paola, is just two: the result of his third marriage, in 1996, to Elena Ochoa, a Spanish doctor of psychopathology (there are four older boys in the Foster family). They live in their vast Foster-designed penthouse above the office, and rent a house in Switzerland. Foster is shaven-headed, sinewy, tanned, clearly quite disgustingly fit.

Willis Faber Dumas, Ipswich, 1975

It is also exactly 25 years since he completed his most famous early building, the curving black-glass headquarters of the insurance company Willis Faber Dumas (now Willis Corroon) in Ipswich: at about the same time as his one-time partner Richard Rogers was, with Renzo Piano, completing the Pompidou Centre in Paris. Willis Faber is now Grade 1 listed, and rightly. With its amoebic shape, huge open floors, grand sequence of escalators and rooftop garden, it was revolutionary, the first "groundscraper". At the time, Foster was working with colleagues who went on to become independently successful: Michael Hopkins, Jan Kaplicky of Future Systems, Ian Ritchie among them. The standard criticism of Foster is that, these days, he is doing nothing so mould-breaking. He's ready for that one.

"Take the Millennium Bridge. That is the first of a new generation of bridges. A bridge that doesn't have either arches underneath it or masts above it. In that sense it is a total breakthrough. Then we're doing the most advanced wind turbine in the world, we're absolutely at the cutting edge of those. One super-sophisticated windmill will power 1400 homes. Or take the Reichstag in Berlin. There is no other building in the world which has zero carbon dioxide emissions, using reneweable fuel and natural ground water cooling, reflecting natural light deep inside. It is an incredible crowd-puller. I'm talking about social breakthroughs, I'm also talking about technological breakthroughs".

Back at the time of Willis Faber, he recounts, he did a drawing where the glass walls curved in over the roof - but had to abandon the idea because it wasn't technologically possible. Today, it is. Hence the Reichstag dome, hence the bulbous new building for Ken Livingstone's Greater London Authority, hence the upended Zeppelin of his controversial City of London skyscraper proposal on the site of the bomb-blasted Baltic Exchange.

Music Centre, Gateshead (project)