Simon Thurley, chief executive of English Heritage, is a man with a faintly embattled air. He managed to keep in the background when his chairman, Neil Cossons, presided over an ill-fated attempt to head off the epidemic of tall buildings that is threatening to turn London into Shanghai-on-Thames. But it is Thurley who is now making the running in what looks suspiciously like a stand for the organisation's credibility by fighting two cabinet ministers (Tessa Jowell and Margaret Beckett) and their attempt to demolish London's Commonwealth Institute. Having failed to stop the skyscrapers, or even to protect Fortress House, the neo-Palladian Savile Row office building which used to be his own headquarters (now threatened with demolition), he has no choice.  He deserves to succeed: the fate of the Commonwealth Institute is one of those turning points in the history of taste, like the Euston Arch, the great Doric Victorian entrance to the station demolished in 1962 in the name of modernisation at exactly the moment that the Commonwealth Institute was being built. It was designed by the dependable but stolid practice started by Sir Robert Matthew, known as RMJM. Inspired by Le Corbusier's Philips pavilion at the Brussels expo a few years earlier, this was the one project with which RMJM decided to kick over the traces. The firm produced a modish hyperbolic parabaloid roof, sitting like a witch's hat perched precariously on a blue box.  It was designed to house the Commonwealth Institute's collection of handpainted dioramas celebrating the finer points of bauxite-mining in British Guiana, tea-planting in Ceylon and rubber-growing in Malaya, which I, for one, remember with a certain fondness from my school days.  When it was new, this was as modern as official London got: a slightly shocking intrusion to the skyline against the backdrop of a Royal Park, with an interior that had something of the flavour of an expo. Of course, that fragile-looking roof leaked almost from the beginning and, as coup followed coup, the dioramas couldn't keep up with changing political and economic realities. The building's present ramshackle state, betraying brave hopes gone sour, is a pretty accurate reflection of the Commonwealth itself.  Even though the government has already poured £3 million into attempting to patch up the building, it is now empty, a victim of policy shifts as well as technical problems. The Commonwealth Institute wants to be allowed to sell the site for development and use the cash generated for educational programmes in the poorer member countries. Vigorous lobbying by a secretary-general determined to play the Africa card, by suggesting that the obscurantist efforts of conservationists are snatching the bread from the mouths of countless million orphans who have never seen inside a school, has persuaded two cabinet ministers to ignore the advice of the Department of Culture, Media and Sport, and back a private act of Parliament to have the structure stripped of the protection of listing. It's a process that is going to divert the Commonwealth's precious resources into the hands of its lawyers rather than the poor.  The building is a reminder of an almost forgotten, but significant moment in Britain's recent history, and its attempt to get to grips with its post-colonial future, presenting the Commonwealth as a viable contemporary alternative to the outdated concept of Empire. On those grounds alone, it is an important monument to preserve, in the same way that Berliners fought to save the former East German parliament building, or a younger generation of Muscovites are struggling to stop the demolition of all the relics of Stalinism by cowboy capitalist developers in league with the mayor.  Apart from its historical significance, the Commonwealth Institute demonstrates that the Sixties were about more than brute concrete. And at a moment when London is going through a period of unprecedentedly rapid change, there is a strong case to be made for retaining as many of the city's defining landmarks as possible.  The case against the Commonwealth Institute rests on the fact that its owners believe that they have no further use for it. That of course is exactly what was once said about the Roundhouse, St Pancras Station, the Royal Exchange in Manchester, the Albert Docks in Liverpool and countless other troublesome redundant buildings. Why not try out a range of other possibilities, even as a temporary market, or exhibition space, until a more secure long-term future can be found for it?  Thurley is vowing to fight, putting both sides in line for colossal legal fees. But he has no option. If a building of this scale and quality is demolished, it calls the whole point of the listing system into question.  The Commonwealth Institute belongs to the same moment in architectural history as the even more decayed, but more distinguished, seminary in Cardross, designed by Gillespie, Kidd & Coia. The seminary now sits rotting in ruins on the edge of Glasgow, covered in graffiti, which suggests that Britain has yet to take the architecture of the Sixties to its heart. But then in the Sixties, the Victorian architecture that is now so valued was just as neglected and unfashionable.  Unwelcome Visitors The newly established London Architecture Biennale, now in full swing, has overtaken the longer established but worthier Architecture Week as the architectural profession's premier charm offensive. It's an attempt to persuade a sceptical world that architecture is neither a private religion, nor a cabal plotting to foist the unbuildable and the uninhabitable on to a hapless public.  One architect who I suspect will not be found mourning over the destruction of the Commonwealth Institute is Quinlan Terry, who is doing a drawing workshop at Riba's headquarters during Architecture Week. It's hard to know which is the least likely: that the profession should invite a man who has devoted the past 40 years to cultivating a distinctly uncordial distaste for its leading lights whom he sees as conspiring to do him down, or that Terry should consent actually to set foot inside the Riba's Portland Place building.  Certainly from the tone of Radical Classicism, the new monograph that Terry will be promoting in the institute, nothing has been forgiven, or forgotten. The Cambridge art historian David Watkin's text couldn't be clearer. 'The modernism with which Quinlan Terry has had to battle is, like the Taliban, a puritanical religion.' But then Riba is facing an even less welcome visitor, in the shape of Peter Phillips, the BNP activist running for president.  Should the Commonwealth Institute be demolished? review@observer.co.uk
There is a strong case for keeping London's defining landmark buildings and Deyan Sudjic is prepared to make it.