What price Fantasy Island?

There is little to suggest what lies on the other side of this sea wall in Essex. A sign by the side of the road simply warns "Danger: Tidal Causeway". As we drive over the hump, the unmistakable smell of ripe seaweed hits us, along with the sight of a slush-covered causeway snaking for a mile across mud flats. In the distance lies a lick of green - Osea Island.

There is little to suggest what lies on the other side of this sea wall in Essex. A sign by the side of the road simply warns "Danger: Tidal Causeway". As we drive over the hump, the unmistakable smell of ripe seaweed hits us, along with the sight of a slush-covered causeway snaking for a mile across mud flats. In the distance lies a lick of green - Osea Island.

Said to be the area's best kept secret, the private island - only a 20-minute helicopter ride from central London - could be yours for an offer in excess of £6m.

For that, you not only get the land, a one-and-a-half mile by half-a-mile strip across the Blackwater Estuary five miles from Maldon and 48 miles from London. Also included in the sale is a 10-bedroom Edwardian manor house with a staff flat, a tennis court, a saltwater swimming pool and an orchard.

There is also a six-bedroom secondary house overlooking the sea, a period farmhouse, 10 further houses and cottages, plus a large Edwardian house and chapel converted into five luxury houses and seven apartments. Along with the sandy beaches, there are deep water and dry moorings, a private quay, a slipway, and grassy farmland.

Living on Osea Island, which has no shops and where no milkman delivers, takes considerable forethought.

"We have to plan ahead a bit because you only have four hours in every 12 to get across the causeway [owing to the tide]," says Hilary Cole, who owns the island with her husband David. "Sometimes we come back by boat or go by boat. I quite like riding home by boat, especially in the summer. It's not such fun in winter, though."

But even the Coles, after 24 years, still occasionally get it wrong. "You drive over the sea wall and see that the causeway is covered, and that's it," says Mrs Cole. "I go and stay with a friend usually, or ring someone on the island and get the boat. We don't have a helicopter - that would be very handy."

While the milkman doesn't include the island in his round, the postman does. "We wondered about asking the Indian restaurant down in Heybridge to deliver," muses Mrs Cole. "They deliver free within a three-mile radius, and we are just within that. I have a feeling they might not."

The Coles have bought the island twice. David and his brother Michael became owners in the early 1960s, paying £70,000. The family held on to it for 10 years, then David and Hilary moved to Devon and Osea was bought by Cambridge University. When, eight years later, they heard it was up for sale again, they made an offer. While now ready to "downsize", their love affair with Osea is far from over.

"It's secluded, nobody can get at you for eight hours out of every 12," says Mrs Cole. "I think it'd be very good for somebody wanting privacy. There's absolutely no sound and it's completely private. You can swim at high tide, sail and water-ski."

The charms of the island are immediately apparent. Rabbits abound, as do butterflies and birds, including every type of British owl. Several thousand dark-bellied Brent geese make a pit-stop here each year.

Osea has been inhabited for at least 5,000 years. There is evidence of Neolithic villages. The Romans built the causeway, a salt works and a pottery. After the Conquest it became the property of William the Conqueror's nephew, and later passed to the Earls of Essex, Sussex, Gloucester and March.

In about 1900 the island was bought by the brewer Frederick Charrington, who tried to set up a temperance mission there. "He'd been walking through the East End of London, and stopped outside a pub where a woman was crouching with her child saying: 'Give me a penny, I'm starving and my child's starving'," says Mrs Cole. "And he looked up and saw the words 'Charrington Ales'. The woman's husband was inside drinking the money away and he thought, 'this is terrible'. He gave up drink then and thought he'd run a temperance house.

"But the Maldon fishermen used to come to buy their stores at a little shop in the island's village, and they'd bring drink and hide it under the hedges. We still find lots of gin bottles with little round marbles in them."

The island was taken over by the Navy in 1918, and became HMS Osea. During the Second World War it was used for the construction of torpedo boats. "Nobody knew it was here, in fact nobody knows it's here now really," says Mrs Cole.

Except, of course, the island's 20-odd tenants, mostly professionals from London, all on yearly shorthold leases. About three live on the island permanently and the rest use the island for weekends and holidays. Annual income from the accommodation is around £135,000, plus a subsidy of £25,000 a year for 200 acres of land registered under the Ministry of Agriculture, Fisheries and Food's Habitat Scheme.

Clive Hopkins of estate agents Knight Frank says interest in Osea is high because of its uniqueness and location. "It's very rare for something like this to come on the market. It's almost unique to find a private island off the English coast, both accessible and secluded. It will appeal to sailors, birdwatchers, artists, photographers and authors through to City business people."

Details from Knight Frank on 020-7629 8171