The future of an island off the Devon coast that has been transformed from the haunt of pirates and chancers into a haven for wildlife and seekers of the quiet life has been secured for another half century.

A fresh 50-year agreement between the charities that own and run Lundy is being signed that will offer protection for the flora and fauna (and the hardy humans) who live on the weather-battered hunk of granite in the Bristol Channel.

“It’s a very special place,” said Dean Jones, the Landmark Trust’s Lundy warden. “There’s something very exciting about being on a small island like this surrounded by nature. You’re in the elements all the time. It’s remote but there’s always loads to see and do.”

Jones’s duties range from looking after the thousands of seabirds that visit Lundy or make it their home, to helping parties of schoolchildren alight safely when the island’s veteran passenger and freight ship, the MS Oldenburg, arrives at the jetty. “The nature of living on an island is that we all have to muck in to make sure everything is tickety-boo,” he said.

Dean Jones, the National Trust’s Lundy warden. ‘It’s remote but there’s always loads to see and do.’ Photograph: Jim Wileman/The Guardian

It is fair to say that over the centuries it has not always been tickety-boo on Lundy.

For hundreds of years the island’s strategic position 12 miles off the Devon coast made it a perfect hideout for pirates who would pillage ships heading into English ports.

Others that have tried to make a quick buck out of it include the 18th-century north Devon MP Thomas Benson, who was paid to deport convicts to the US – but instead dropped them off 4,000 miles short on Lundy.

More than 100 years later a businessman called Martin Coles Harman bought the island and declared himself king of Lundy. He was fined by the House of Lords for setting up his own currency, with “half puffin” and “one puffin” coins.

However, a new chapter in the island’s history began in 1969 when the late Jack Hayward, the former owner of Wolverhampton Wanderers football club, bought Lundy and donated it to the National Trust, who leased it to the Landmark Trust.

Since then it has enjoyed five decades of stability and prosperity that would turn failed profiteers of the past green with envy. This weekend, no matter what the weather, a party will take place to mark the signing of a new 50-year agreement between the two charities.

Looking out from the top of old Lundy lighthouse. Photograph: Jim Wileman/The Guardian

More than 18,000 visitors every year undertake the often choppy crossing from north Devon or hop on to the winter helicopter in search of a taste of the quiet life. Some stay for a few hours, others for a week or more in one of the island’s houses.

Lundy has become a magnet for divers, climbers, even campanologists who ring the bells of the surprisingly large Gothic-style St Helen’s church. There is one shop and one pub, the Marisco Tavern, but people find plenty to fill the hours.

Sue Waterfield, who runs the shop, said: “It’s a fantastic place to get away from the hustle and bustle of the mainland. The 4G is poor and there’s very little phone signal so you get away from all that.”

Sue Waterfield, who runs the one shop on the island. Photograph: Jim Wileman/The Guardian

Even as the number of visitors has climbed steadily, the seabird population has soared, trebling over the last 15 years to more than 21,000. At the turn of this century, there were fewer than 10 pairs of puffins on Lundy. Now there are about 7,000 of the birds.

Other important conservation work has included clearing the island of rhododendrons, which has allowed the native Lundy cabbage to flourish. In 2010 the sea around Lundy was designated as the UK’s first marine conservation zone. The waters around the island are alive with grey seals and lobsters.

Restoration and repair work has also taken place on the church, road, jetty and the 23 self-catering properties.

But there are challenges. The island’s custodians fear the climate emergency could endanger the wildlife – for example, rising sea levels could result in fewer places for seals to haul themselves out of the water.

The island’s green credentials are let down by its power source – diesel generators – and plans are afoot to work towards renewable energy, though few are keen on turbines being set up on the cliffs. Work is under way to make sure the island does not run out of fresh water and the ageing ship will need replacing soon.

“Fifty years ago Lundy was on a knife-edge,” said Derek Green, the Landmark Trust’s manager of Lundy. “It’s been a fantastic journey and we’re thrilled to be signing the new lease. The island offers a rare experience: large enough to have a life of its own, which visitors can share and enjoy, but small and far away enough to be a world apart.”