Scenically, it suggests something out of a Gabriel Garcia Marquez novel: before me, spooky yet magical, is a sprawling Portuguese plantation house. Derelict and desolate, after years of neglect it is wrapped in the rampant vegetation that covers São Tomé island, in the Gulf of Guinea west of Africa.

Vines loop around the veranda's broken balustrade. Moss grows between stone lintels in roofless rooms. In the grounds is a curious retro swimming pool, cracked and empty, on a lawn reclaimed by jungle fern. Yet with a little concentration, I can picture it half a century ago in colonial times, when the Portuguese cocoa baron and his family ruled the roost, commandeering African workers on this bottle-green equatorial outcrop.

'If roça houses like this were restored as luxury hotels, tourists would come and stay, right?' asks Mario Almeida, who is helping me discover São Tomé. Without doubt they would. The colonists may have been brutes, but they had an eye for sublime spots on which to build their cool, thick-walled homes. From this property, Roça de Porto Alegre, the view is spectacular: in one direction, a cobalt bay and white inlets; in the other, the looming pinnacles of extinct volcanoes. Some neighbouring roças have already begun welcoming overnight guests as word of the hidden pleasures of São Tomé and its sister, Príncipe, spreads among European travellers. Little by little, these twin islands - Africa's second smallest country - are drawing a growing trickle of travellers with a passion for forgotten tropical outposts.

The flight, via Lisbon, takes you to the drowsy capital, São Tomé, where life unravels along pot-holed avenues lined with lofty palms and flaming red Erythrina trees. There's a photogenic yellow cathedral, a garish pink presidential palace, a harbour-front of crumbling quays and cargo ships, and town houses with facades tiled in azulejos, the blue-and-white Portuguese tiles.

Time and again, I am taken aback by echoes of the old country: the diminutive squares paved with black and white wavy shapes that would be perfectly at home in Oporto or the Algarve; the milky galão coffee served in long glasses, and the sticky pasteis de nata cakes at waterfront Passante café, part of Hotel Miramar.

In the narrow, cobbled district around the central market the place feels more like the Caribbean. In sight of stone pillories where slaves shipped from West Africa were once bought and sold, reggae and Brazilian samba tunes blare from the radios of decrepit yellow communal taxis. There is busy bartering for pungent spices, even more pungent dried fish and rough red wine that comes in five-litre garafao flagons. A woman moves by with a huge tray of breadfruit on her head; another sells cloth woven into mesmerising patterns.

São Tomé is only 40km long but it feels much longer on the lurching minibus journey south. The route, along the unmade coastal road, is taking me to Ilhéu das Rolas, an islet reached by speedboat, just off the southern tip of São Tomé. There, white surf pounds empty beaches of fine sand, strewn with driftwood and coconuts. En route, the road twists around cliffs hung with foliage, over rickety bridges spanning muddy rivers. We pass wooden shack villages where rows of flying fish dry on bamboo racks and fishermen chisel dugouts from oca trunks. You can easily imagine that the volcanic islands of the Caribbean - St Lucia or Grenada - were something like this 5o or so years ago.

Rounding a promontory, the driver points to a scattering of rocky reefs out to sea: the Sete Pedras, piercing the water's blue surface like broken teeth. According to local legend, a huge slave ship, the Angolares, was wrecked here in the 16th century, its survivors washed ashore. Their descendants are today's fisher folk in the village of São João dos Angolares, communicating with a unique Creole that indicates their origins. Other survivors were less lucky. Re-enslaved, they were put to hard labour on the plantations of sugar (later coffee and cocoa), breaking their backs for the Portuguese masters who dominated these islands from 1470 until 1975.

Times are slowly but surely changing: on Ilhéu das Rolas, a luxury resort, the Pestana Equador, has just opened and now, 30 years after they fled their African colonies for good, the Portuguese are back, along with other Europeans. They're here with fistfuls of euros to spend on air-conditioned bungalows and chilled papaya juice served to them poolside on their loungers. It's a beautiful, if slightly removed, experience.

Back on the mainland, in from the coast, I watch São Tomé's cloud-cloaked interior unravel through the glass of a 4WD in the company of Luis Mario. Progress is slow, but after visiting the derelict plantation house Roça de Porto Alegre, we find ourselves in a surreal mountainscape where curtains of silver mist suddenly lift to reveal bulbous humps hung with tresses of wild green vegetation, phallic shafts of magma rising skywards.

This is the land of the plantations that once made São Tomé the world's largest producer of cocoa. Luis Mario's forebears were serviceis - or forced labourers - who were brought here from Angola and Cape Verde to work, with thousands more, on sprawling roças such as Bombaim, our destination today. In their golden age, these roças had their own hospitals, railways, even bullrings. But starved of management or capital after the bosses had fled the island, much of Bombaim has capitulated to the forest - a row of Portuguese administrators' fine houses remains, enveloped in green tendrils, beside rusting lines of long-defunct tractors.

A few workers eke out an existence among the outbuildings, earning a living from small plots. As I inhale the scents of wood smoke and frying fish on the jungle air, a mother sings a lament to her baby boy while she cooks over an open fire. But there is reason for optimism: Bombaim's main plantation house, Casa do Patrão, has recently been rescued from the jungle and half-restored by new Portuguese owners to offer simple rural tourist accommodation.

Plumbing and electricity are vestigial, but there are five bedrooms, a shared bathroom and cold showers. We dine on delicious calulu (a spicy-hot stew of fish and breadfruit) with cold Sagres beer imported from Portugal, then we retire to the veranda to watch swarms of fireflies, listening for anonymous thumps and screeches emanating from the forest.

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