Luckily the political delays meant that, when it finally got the go-ahead, computing techniques had arrived to help the design. There was just - and only just - enough primitive computer power in Sydney at the time to perform the intensely complex structural calculations needed. Utzon and Arup inevitably fell out, but were eventually reconciled. The explosion of "high-tech" architecture in the 1970s - a completely different style - was made possible by Sydney, because it showed how architects and engineers could collaborate to produce extraordinary new kinds of buildings.

But Sydney is remarkable for another reason: it is a complete one-off. It does not fit into any stylistic or chronological category. None of Utzon's other buildings - churches, government departments, houses - looks anything like it, and those architects today who try to copy his concept always end up looking very second-rate indeed. It is 'modern', certainly, but it is an expressive modernism that was quite at odds with the rectilinear "international style" of its time. It has more in common with the work of the American maverick genius Frank Lloyd Wright, for whom Utzon once worked briefly. Of course, its location is an enormous help, sitting as it does on a promontory with water on three sides and the famous Sydney Harbour bridge as a picture-postcard backdrop. But Utzon masterfully exploited the site as no-one else could.

The building can lay claim to being the first multi-purpose "performing arts centre", with two main auditoria, but in truth its function is wholly unimportant. Its plinth, containing the bulk of the working space, is completely subservient to the geometric roof shells with their carapace of glittering tiles - creating cowled forms, from which cascades of tilting glass facades precipitously emerge. Utzon left Australia in high dudgeon in 1966, never to return, before he could finish designing the interiors. Today there are belated plans to improve the inside spaces in consultation with its now octogenarian creator.

As with Sir Christopher Wren at St. Paul's Cathedral, Utzon was humiliated and removed from overseeing the final stages of his masterwork. But for all his manifold difficulties, what other contemporary architect can claim an equivalent achievement? Who else, in the post-war years, has designed a building so universally recognised and admired? Those who sometimes criticise architects for creating sculpture rather than workable buildings should look at Sydney and reflect: here was a moment when the sculptural solution was triumphantly correct. With the Sydney Opera House, modern architecture finally came of age. It showed us that anything is possible: and it demonstrated that sheer, seductive beauty for its own sake is nothing to be ashamed of.