Penzance's lido is a Modernist marvel. Stephen Wood dips into a pool of restored glory
The morning was grim, clouds forming, a gloomy dome where the sky should have been. The view was grim, too. Below the promenade, the concrete semicircle of a derelict lido pushed out into the sea. In the middle of the pool's stagnant water stood a vase-shaped fountain, long dead, its old blue paintwork stained with rusty streaks.

Having spent half the night driving to the West Country - and the rest half-asleep in the back of the car - I was determined to investigate the lido, which opened in 1935 and closed 60 years later. Leaning over the promenade railings, I could see that it would not be a problem to get in; as I looked down, a woman about the same age as the lido (but in much better condition) walked across the apron in a bathing costume, pulled on a rubber cap, waded into the murky water and began to swim around the perimeter.

I followed her route down the steps from the promenade, pushed open the unlocked gate with its "No admittance" sign, and walked on to the poolside. Here the view was better - and worse. The two fountains flanking the steps into the pool were in an even sorrier state than the one in the middle; but the rather plain, Moderne-style building housing the ticket office and changing-rooms had charming decorative friezes along the lip of the "Sunbathing Terraces" and on the porches.

With a lick of paint, a few more customers and some sunshine, it would have been wonderful. Instead it was just depressing. Luckily this lido, Plymouth's Tinside pool, was not the one for which I had driven down from London.

The year in which the Tinside pool opened was the height of Britain's lido boom. The London County Council led the way: having already approved the building of pools in the Victoria and Brockwell parks, it announced a programme for seven more. That year pools also opened at Ilkley, Norwich, Peterborough, Saltdean and Aylesbury.

Partly this growth was a response to demand, caused by the twin Twenties crazes for sunbathing and cross-Channel swimming. But it was also motivated by public concern for health and fitness: an Army recruiting drive that year revealed that only 38 per cent of applicants could meet the minimal requirements of the physical test. Hence at the grand opening of the Jubilee Pool in Penzance, in May 1935, Alderman Treganza stressed the health benefits it would offer: "There can hardly be a better form of bodily exercise than swimming ... all the muscles are brought into action," he proclaimed (to shouts of "Hear, hear", according to the report in The Cornishman).

The lido boom lasted until the Second World War. Thereafter, rival attractions (including, ultimately, cheap package holidays) sent Britain's big open- air pools into terminal decline. In 1991, the Thirties Society published a booklet, Farewell My Lido, which reported that despite the "delightful and evocative architecture" of these period pieces, budget-cutting in the Eighties had meant that "only a handful still survive, and ... none is free from threat". From its survey, it picked out a few as case studies. Tinside was not among them ("No threat of closure" was the judgement then); the lidos at Finchley in London and Penzance's Jubilee Pool were both being judged to be "in the balance".

Finchley's two pools - one an extravagantly styled children's pool with a cascade flanked by arcades and lion's-mouth waterspouts - have now disappeared, along with the sunbathing lawns. In their place is a Nineties leisure centre including, along with fast-food outlets, shops and multiplex cinema, two pleasant new indoor pools and a pitifully small outdoor swimming-bath.

At Penzance, the balance has swung the other way. The huge Jubilee Pool has not only survived, but is in a sensational condition. What The Cornishman judged at its opening to be "a work of art" (hear, hear) has been restored so beautifully as easily to justify driving half the night to see it.

When I arrived in Penzance, the weather was as stunning as the view. Beyond the delicate entrance arch, at the eastern end of the promenade, sunlight sparkled on a million gallons of sea-water filling the Jubilee Pool. The paintwork around the pool was ultra-ultramarine, putting the surrounding Mounts Bay to shame. The pool's almost-triangular shape juts out into the bay. Its decision was inspired, so the architect claimed, by the sight of a seagull landing on the sea; but the pool in fact looks like the bow of a great ocean liner (a more suitably Modernist image, since one of Le Corbusier's early inspirations was the design of liners).

The architect, Capt Frank Latham, Penzance's borough engineer, built the pool walls using existing rocks as its foundation. As a result, the design combines the hard edges of Modernism with, along the sea walls, the natural curves of the rocks. The flat, rendered surfaces of Modernist buildings always look terrible when the paintwork is worn, superb when it is fresh: the Persil-wash white on the Jubilee Pool's sea walls was so dazzling that their subtle contours would have been elusive but for the shadows cast by the afternoon sun. The beauty, as breathtaking as that of the Alps or Milan's San Siro football stadium, made me want to do something about it. So I did. The water was breathtaking, too.

Braced, I retired to the Yacht Inn across the road. Also a Thirties building (apart from the new plastic window-frames), this was the perfect place to stay. From my bedroom at the front of the building I could see the Jubilee Pool last thing at night and first thing in the morning.

The following day was a Saturday. I paid my pounds 1.50 entrance fee (a bargain: what other Grade II listed building charges so little, and lets you swim in it?) to join the crowd of noisy children inside. On a good day, the Jubilee Pool has attracted as many as a thousand paying customers since it reopened in 1994, and this day was looking good. The water, topped up at high tide through the sluices (which now have grilles on them to stop fish getting into the pool), was still bracing; but that only added to the volume of the yells as children flew down the water-slide into the depths. The atmosphere recalled childhood memories of Finchley lido, the only rogue element being the sound of the seagulls.

"Rogue" is the right word. By midday, the trickle of people bringing chips back from the caravan cafe parked up by the promenade had developed into a stream. One family unwisely offered a couple of chips to a seagull; a flock of colleagues descended, and - like a scene from Hitchcock's The Birds - tried to beak the whole lot. Luckily, their three-year-old knew what to do, charging at the gulls to reclaim the family chips.

I spent the day swimming a few lengths (a geometric puzzle in a triangular pool), sunning myself on the whiter-than-white concrete surrounds, eating chips and admiring the magnificent, German-quality fittings in stainless- steel - a precaution against the corrosive effects of sea-water.

Ten years ago this lido was heading the same way as the Tinside pool 80 miles up the coast, or towards a fate worse than death - that of conversion into an indoor "water-fun" leisure centre beneath a huge copper pyramid. Its survival is largely thanks to John Clarke. A self-effacing man (he takes pains to pass on the credit for saving the Jubilee Pool to others), Clarke had been assistant county architect for Cornwall; on his retirement in 1984, he took an interest in local planning affairs - notably the proposed redevelopment of the then-decaying Jubilee Pool.

Despite local opposition to the proposals, the 1990 draft plan for Penzance - to Clarke's displeasure - still zoned the pool site for a leisure development. He decided to object. "I thought lots of people would be objecting," he says. "But I was the only one. Not many people read local plans; they're rather indigestible documents." He pressed for a public inquiry. "I said I'd call witnesses, and have the issue ventilated, although I'd never done anything like that before."

The district council ultimately abandoned the development and, to its credit, decided instead to devote pounds 280,000 to strengthening and refurbishing the pool. Meanwhile, Clarke turned his energies to protecting its future by seeking to have it listed. "Because a lot of other pools had sadly closed, this one was becoming a rare item. And, as anyone who has watched The Antiques Roadshow knows, such rarity has the merit of drawing attention to an object's value." In March 1993 the Jubilee Pool was designated a Grade II listed building. Immediately grants materialised, from English Heritage and the European Regional Development Fund among others; ultimately, the council had to pay only pounds 20,000 of its bill for the work on the pool.

A Jubilee Pool Association was formed, with John Clarke as its president. More funding was raised, from corporate sponsors including the South Western Electricity Board: "I wrote them the best letter I've ever written," says Clarke, "and the two pages were worth pounds 2,500 each." In May 1994, a grand reopening saw the pool start its new life. Soon, if lottery funds are forthcoming, it will have a poolside cafe. Its design, approved by the council, is the work of John Clarke.

The Jubilee Pool. Penzance, (01736 369224) is open daily, 1lam-7pm, until early September. Admission charge for swimmers: pounds l.50 for adults, pounds 1 for children. B&B at The Yacht Inn, the Promenade, Penzance (01736 362787) costs pounds 25 for a front room, pounds 20 for a berth at the back.