he Serpentine Gallery in Kensington Gardens has long been one of London's most attractive exhibition spaces. In a delightful building, free of entry and easily accessible in a public park, its commitment to contemporary art has given it a permanent and loyal clientele - in addition to some habitual users of the park who use it to escape from the rain. With shows changing every six weeks or so, it traditionally keeps up a punishing schedule. Previous exhibitions, such as Man Ray and Jean-Michel Basquiat, and Tilda Swinton lying asleep in a glass case, have had queues stretching out of the building into the gardens beyond.
During the past year, the gallery itself has been firmly closed, its aspirations represented by a limited programme of sculpture shows on the lawn outside. Beneficiary of a pounds 3m windfall from the National Lottery, the 1930s building - once a teashop - has been remodelled, re-equipped, turned inside out and back-to-front, and then made to look much as it was before. Now it is about to re-open.
Julia Peyton-Jones, 45, a curator with wide experience of the art world, has been the gallery's director for the past six years, and is the presiding genius of its metamorphosis. She emphasises the significance of its re- opening. "We will be the first large-scale, Lottery-funded, Arts Council project to be realised in the capital under the new government."
As it happens, there is more riding on the gallery's reopening than that. After various public-relations disasters, of which the long-running saga of the Royal Opera House is but one, the troubled Arts Council needs a success story to show that it can choose its projects for Lottery funding wisely, and keep an eye on their successful completion. The Council is hoping that the Serpentine Gallery, recipient of the second tranche of Lottery funding, will come up trumps.
At stake too is the future success of one of the triumphs of the Thatcher era. During the long period of Tory rule, two significant developments occurred in the art world. Public funding was scaled down, and galleries were left to fend for themselves, experiencing the magic of the market at first hand. At the same time, Charles Saatchi, Tory millionaire advertising entrepreneur, personified the transformation of the public's appreciation of contemporary art - a process through which it has become almost as popular as rock music.
The Serpentine Gallery has been a showcase for these developments. Originally set up by the Arts Council in 1970 to exhibit the work of young artists, it was given a modified form of independence from the state in 1987, just as the change in the public mood was about to take place. While still in receipt of some public money through the Arts Council and Westminster City Council (providing 40 per cent of its annual income), the gallery's trustees are obliged to look to foundations, businesses and wealthy benefactors, to make up the shortfall. The gallery's annual turn-over is close to pounds 1m. In other areas of the economy, the process became known as privatisation.
The formula proved highly successful. In the era of the Young British Artists, the gallery has been a glamorous venue, a social success that has brought in sponsorship money on a substantial scale. Unlike those of some less privileged projects, its books balance. Now it faces a new future in a splendidly refurbished building that will certainly prove expensive to run.
The first thing an old-time visitor will discover is that the way in is now on the south side of the building, facing towards Kensington Gore. What used to be a small yard for the dustbins is now the entrance hall, with space for the gallery's bookshop, and with offices tucked in above. This is the most obvious change, and in some ways the least successful. In the old days you came in directly from the park through a decorous portico on the north or through the immense French windows on the east side. Now you are faced with a more formal entrance, flanked by forbidding slabs of grey brick.
Inside the galleries, many people may not even notice the small differences. There are more roof lights, and although the northern portico has been blocked in - to create an education studio - the central gallery has been given two additional internal entrances by way of compensation. The most significant changes, and the most expensive, are the ones that most people will not see: a gigantic air-conditioning operation, housed in the excavated basement; a vastly reinforced wall frame that will permit the heaviest Richard Serra girders to be hung; an entirely new set of double-glazed French windows; and a small suite of offices on the floor above.
The authorities at English Heritage forbade any significant changes to the Grade Two listed building. Wouldn't it have been cheaper to have started from scratch, I ask John Miller, the architect responsible for the gallery's refurbishment. "An entirely new building would probably have been more expensive," he answers evasively, "and in this case, there was no question even of an extension. We had to stay within the footprint of the existing building."
In the 1930s, on the last occasion that anyone gave much consideration to the buildings in Kensington Gardens, the views of the Futurists were more hegemonic. The existing tea-shop on the site was ruthlessly pulled down and the new Serpentine tea-shop was built that year to the designs of J Grey West, chief architect to His Majesty's Office of Works.
The Serpentine lake was opened for bathing by a Labour minister, George Lansbury, in the early 1930s, prompting A J P Taylor to comment caustically that the Serpentine Lido was "the only thing that keeps the memory of the second Labour government alive". In 1970, it was another Labour minister - John Silkin, the minister of works in Harold Wilson's government - who ordered J Grey West's park tea-room be turned into an exhibition space for contemporary art.
Seeking some folk memories of the old days, I visited the gallery's first director, Sue Grayson Ford, who now runs the Cardiff Bay Arts Trust. "Silkin was very worried at that time - this was the end of the Sixties - about squatting. The squatters were already beginning to move along Park Lane. What would happen if they took over the Serpentine? Silkin identified its potential as a gallery, and asked the Arts Council if they'd like to take it over." Grayson, an Arts Council regional officer, was sent to investigate. "I got the keys, and it was love at first sight. I knew that there was a dearth of spaces for young artists to show, and I argued the Serpentine could be cheaply converted." For six months in 1969, she bullied and cajoled the Arts Council into taking on the Serpentine. "Suddenly there was agreement: the Ministry of Works would convert the building, and the Arts Council would run it."
The Serpentine's special atmosphere was created at the start. "The original idea was only to show artists under 35. The staff I employed were all artists. I knew that was the way to do it, to have people who would be in sympathy with the shows, not like the warders at the Tate." The emphasis on young artists did not survive. The gallery proved to be too popular, and curators began to eye it jealously. The critic David Sylvester thought that the Serpentine Gallery was a lovely space and should be used for big names, not just for unknown young artists. As an influential voice at the Arts Council, he soon got his way. Henry Moore, Andre Kertesz, Giacometti - there was a succession of large and popular shows by famous names. This strategy has been followed by Peyton-Jones. The success of the Man Ray exhibition in 1995 gave considerable impetus to the notion that the gallery needed a secure, air-conditioned building that could mount such mini-blockbuster shows.
Over the past decade, the "privatised" Serpentine has been an envied model. Yet it is hardly typical; it was born with a silver spoon in its mouth. An anecdote in Roy Strong's recently published memoirs tells of an Arts Council meeting in 1987 at which the chairman, the ubiquitous William Rees-Mogg, declared his determination that the new Serpentine should not go down the "ideological" road taken by the Tate, once the home of Carl Andre's controversial bricks. Lord Gowrie, it was argued - as a former Tory Arts Minister and the chairman of Sotheby's - would be a suitable candidate to ensure that the newly privatised gallery would function satisfactorily.
So with Lord Gowrie as the first chairman of the trustees and with the Princess of Wales as patron and with the gallery nicely situated in the Tory enclave of Westminster, no one was going to allow the experiment to fail. Julia Peyton-Jones, who had been the Arts Council's first "sponsorship officer", was an obvious choice as director to take the gallery into the new privatised era, in which the need to secure funds from outside the state sector was paramount. She took over at the Serpentine in 1991.
"The glorious times when we were not reliant to such a degree on additional funding, were a different era," she says. "It was to do with quality of experience and scholarship. It wasn't tested by numbers of people through the door, or column inches." But when all that began to change, she argues, "and when that change was perceived to be permanent, it released an extraordinary amount of energy. It has been incredibly liberating. We were forced back on our own resources - commitment, hard work, energy, enthusiasm, and an awful lot of lateral thinking. I think that's what the Thatcher government did."
The staff at the Serpentine are expected to keep a weather-eye open for changes in fashion and style. Even when shut for a year, they have certainly been busy. I should know: my wife runs the education department. A public art gallery at the end of the 20th century is no quiet temple of reflection; it has some of the atmosphere of a newspaper office or an advertising agency. The best time to get hold of the director is at eight in the morning. Gallery life is about contacts and timing, about publicity and marketing, about flexi-time and the cutting edge. The Serpentine may now be perceived as the very model of Blairite modernisation.
The gallery will finally reopen to the public on Saturday 28 February with an exhibition of the work of Piero Manzoni, an important member of the Italian avant-garde of the 1950s. Although he died 35 years ago at the age of 29, Manzoni's conceptual work - including tins of his own excrement originally offered for sale at the fluctuating price of gold - lies clearly in the tradition followed by today's generation of Young British Artists which the Serpentine has done so much to promote.
Piero Manzoni: Serpentine Gallery, Kensington Gardens, London SW7. 28 February to 26 April. Daily 10am to 6pm. Admission free. Details 0171- 298 1515.