THERE is a moment in her memoir where Vanessa Branson is describing how she came to be the reluctant owner of a Scottish island.

It was late 1994 and her then-husband Robert Devereux was – in Branson's words – experiencing a midlife crisis. The couple were on their way to visit friends on Mull when Devereux suggested a four-hour detour to view Eilean Shona in Loch Moidart, up for sale with a £1.3million price tag.

Branson, heavily pregnant with their fourth child, recalls grumpily disembarking from the Fort William Motorail. Recounting these events in her new autobiography, One Hundred Summers, she doesn't mince her words.

"Buying Eilean Shona was an act of lunacy," writes Branson. "Over two thousand acres of land crying out for attention, with 10 cottages in various states of disrepair, crumbling outbuildings and an 11-bedroomed main house with a leaking roof and an ancient boiler."

Yet, the art dealer and hotelier, who is the younger sister of entrepreneur Sir Richard Branson, instantly recognised its magic – the same enchanting landscapes that had captivated author JM Barrie when he spent a summer on Eilean Shona while writing the 1924 film adaptation of Peter Pan.

After a short boat journey, she and her brood stepped onto the island's mossy shore. In that moment, it was a done deal. "We were bewitched," she writes.

Fast forward 25 years (give or take a month or so) and Branson, 61, is speaking to me from her Sussex farm. It's late May and lockdown has meant it's been some time since her last visit to Eilean Shona which, over the past quarter of a century, she has painstakingly transformed.

"We were going to have a Branson family clan gathering last week," she laments. "Every other year all of us get together there. We love it. It is horrible not being able to go. But we can do it again another time, so all is not lost."

Her love for Eilean Shona leaps off the pages of One Hundred Summers. But that's only part of the story. The book is a compelling blend of memoir and family history, lifting the veil on how the other half live in jaw-dropping detail.

Far from being merely a jet-setting whirl of property portfolios and private islands (although there is plenty of that, too), Branson lays bare a life blessed with privilege, luck and derring-do, yet also with its share of pain and tumult.

A passion for nature and the outdoors was ingrained during a childhood in rural Surrey where her parents owned Tanyard Farm. Despite this quintessentially English upbringing, there are Scottish branches on both sides of the family tree.

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Branson's paternal grandmother, Joyce Mona Bailey, was born at Invergloy House on the banks of Loch Lochy, Inverness-shire, in 1890, while her maternal grandmother, Dorothy Constance Jenkins, arrived into the world at Inverleith Row, Edinburgh, in 1898.

Dorothy's father, Reverend Charles James Jenkins – a man hailing from seven generations of clergy, including two bishops – barely had time to congratulate his wife, Mary Eve, on the birth of their daughter before dashing off for the 7am matins service.

"I have quite a lot of fine Scottish blood surging through my veins," says Branson. "These amazing women. My mum's mum [Dorothy] was born in Inverleith and very proud of her Scottishness. She was focused on education and had a no-nonsense, practical and pragmatic side.

"Joyce Mona was much more ethereal, shy and interested in nature. She never went to school yet knew the name of every moth, wildflower and bird."

It was the discovery of old photographs that sparked Branson's foray into genealogy. "I hadn't shown much interest in family history before," she admits. "It was a bit of a yawn, really.

"Then I had found these albums where only the dogs were identified – there were no names for any of the humans. It was that classic English family thing where Patsy and Daisy had names, but all the people were anonymous.

"When I started fitting the bits together, seeing the different personalities and realising I could identify them, it all became so interesting. They are a pretty amazing lot of people and that made me feel good, to know that I came from such a strange and marvellous heritage."

Initially, it was merely a hobby. "I didn't plan the book at all," she says. "It wasn't planned even as a memoir. I just started writing and carried on. I got to a point where I was writing about my parents and realised it was exactly the centenary of the First World War ending.

"I thought: 'Oh, it is 100 years since my father was born.' I decided if I was covering 100 years, I wasn't going to call it One Hundred Winters because that's a bit depressing. Life happens in the summers. And life is very short. You only have so many summers."

Before becoming a hotelier – Branson also co-owns boutique Moroccan property El Fenn – she carved a career in the art world, running the Vanessa Devereux Gallery on London's Portobello Road and founding the Marrakech Biennale.

Her book has star-studded anecdotes featuring a Who's Who from the glittering international art scene; be it hanging out at the Palladium nightclub in New York with Andy Warhol or having Turner Prize-winner Grayson Perry design an eye-popping pot for her mantelpiece.

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Yet, Branson makes no bones about how, for many years, she was unable to shake the creeping feeling of being an imposter within the rarefied world she inhabited. As a reader, it is almost impossible not to warm to her gumption and heart-on-sleeve honesty.

Branson found a passion project in Eilean Shona. To hear her speak about it now is in stark contrast with the initial sinking feeling she describes when buying it was first mooted on that trip in the mid-1990s.

The sale went through in 1995 – the day before her youngest son Ivo was born. A month later, the couple and a group of friends celebrated Devereux's 40th birthday on Eilean Shona, shucking fresh oysters on its white sands and dancing the night away in the village hall.

The following year, these happy memories were marred by betrayal when it emerged that her husband was having an affair. The couple separated and then divorced, although remained in each other's lives. Branson finally bought out Devereux's half of Eilean Shona in 2018.

"But it is really interesting," she interjects. "Because you can't really own an island. It is not a feeling of possession at all. It is hard to communicate that.

"It is a bloody big responsibility I have taken on to steward it through my tenure and it is something I take seriously. We have been investing a lot of cash and energy into planting trees and keeping the rhododendrons down.

"It is a nature reserve and about preserving its beautiful self rather than turning it into a commercial enterprise. It is an act of insanity and passion having the responsibility of a place like that."

Eilean Shona has won her heart. "Scotland and that west coast gets under your skin. Once you spend time up there, you've had it, I'm afraid."

Yet, her romanticism is often tempered by realism. One of the most powerful sections in her memoir charts the fateful afternoon that, with friends visiting, Branson proposed they all swim back to the main house from Shoe Bay, the island's idyllic blue lagoon and beach.

While she was no stranger to taking a dip in Scottish waters, the choppy waves gave cause for concern. Branson, though, was determined and waded into the icy cold sea loch. Remarkably, she completed the two-mile swim, but not before the freezing temperatures took their toll.

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Arriving at the jetty, Branson passed out. She credits her survival to the quick thinking of her daughter Florence, who ran to the house, ordering her father to fill a bath with hot water. A friend followed behind, carrying a floppy-limbed Branson in his arms.

She recalls lying in the bath, aware of Florence on top of her, shouting: "Come back, Mummy, don't leave us." Eventually the hot water, alongside the restorative powers of porridge and tea, worked sufficiently to get some warmth into her core.

Afterwards, Branson had an epiphany. The swim, she surmises, had been a way of trying to prove her mettle to Devereux. But Branson realised it was time to draw a line. That clarity was a catalyst for moving on. The end of an era. And the beginning of a new chapter.

"When life gets that out of control you have to say, 'Right, it's time to get a grip.' Doing that swim was foolhardy and not something you should do as a mother of four children," she says. "It was foolish. I thought: 'Things aren't right. You are out of gear with life. You need to address this.' It was a turning point."

Today, Eilean Shona is marketed as a luxury retreat with eight cottages and the main house available for rent. The refurbishment of the island's properties has taken many years and has been a chequered journey.

Among its renovations was that of the former schoolhouse. Branson discovered, through a chance correspondence with the film director Joel Coen, that the building's design was by renowned Highlands architect Alexander Ross.

"I had contacted Joel about being part of the Marrakech Biennale and he replied saying he couldn't make it, but he and his wife Frances McDormand were really interested in our old schoolhouse because they love Alexander Ross's architecture.

"At the time, it was abandoned, with the roof falling in. I felt so ashamed that I had allowed this jewel of a building to fall into disrepair. I borrowed money off my London house, mortgaged it and did up the old schoolhouse. It is absolutely beautiful now."

She laughs when asked about the challenges of the project. "There is always a story on Eilean Shona and it is never straightforward. As we were starting the building work, a bat decided to nest in the rafters. We had to wait for the bat and its young to leave, so that held up the building work for six months."

That pales next to some of the other trials and tribulations. Swathes of ancient woodland were felled by hurricane-force winds, with a bout of larch disease another blight on the beleaguered trees.

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Then there is the havoc wreaked by climate change where, after three months of drought, the tinder-dry undergrowth caught fire, and Eilean Shona was engulfed in flames.

"I think that was 2011," says Branson. "It hadn't rained for months. Two-thirds of the island was burned very badly. The worse thing was we had just planted 120,000 hardwood trees. It was awful.

"In retrospect, what we should have done was have controlled fires before we planted the trees. There was so much undergrowth. We got insurance money for it, replanted them all and they are going strong. It set us back a bit and was a shame, but you would hardly know it happened now."

The history of the island is fascinating with the 1851 census detailing the evacuation and emigration of 37 families from Eilean Shona to the mainland in the wake of a potato blight.

During the mid-19th century, the main house was a small hunting lodge owned by a Captain Swinburne who collected myriad tree species on his travels and established a pinetum.

Another notable owner was Lady Howard De Walden, who was given Eilean Shona as a wedding present by her future husband in the 1930s. The De Walden family made numerous improvements to the island's grounds and gardens.

Eilean Shona changed hands twice – the Digby Vane family bought it in 1962, followed by the Steads in 1982 – before it was sold to the Devereux-Branson clan.

"There have been some very eccentric people who have lived there," attests Branson. "But it is an island that isn't bought and sold a lot, unlike others that wear people down very quickly. In the last hundred years it has only had four owners which isn't bad going."

The writer JM Barrie enjoyed summer holidays on Eilean Shona in the 1920s, visiting with his foster sons Michael and Nicholas Llewelyn Davies, and while there is said to have written not only the screenplay for the film adaptation of Peter Pan but also the ghost story, Mary Rose.

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Branson has dubbed Eilean Shona her own Neverland. "I don't know whether you think it's a bit corny?" she asks. "It is amazing because there are days without end on Shona.

"In the summer, it never gets dark. You can be on the beach at 10pm and it's this wonderful feeling of being a child again because you never have to go to bed. You can lose yourself, there are no time constraints.

"Likewise, in the winter evenings, you have time to talk to people. I get that feeling of Neverland when I'm up there. It is fun, you can point out the blue lagoon [at Shoe Bay]. It is a fantasy place and touches the child in you."

The three-and-a-half-mile-long island and its surrounding waters are home to abundant wildlife such as sea eagles, otters, pine martens, red squirrels, deer, dolphins, seals, and basking sharks.

"Whenever I go on a walk it is like going on a safari," she says. "The sea eagles are brilliant. We didn't introduce them. They came about five or six years ago and are reproducing every year. They are described as 'barn doors' and you do feel this amazing presence swirling around above you.

"We are seeing more dolphins. The basking sharks are magical. The sea otters, too, seeing them swimming through the water and leaving that v-trail behind them.

"I know that deer are very destructive, but they are lovely. As long as they keep out of our new woodlands. They swim in – they are canny beasts. One of our cottages, because building work had to stop due to Covid-19, has been empty for spring and a family of pine martens has moved in."

There's a photograph in One Hundred Summers of Branson with her brother Richard, sister Lindy and mother Eve on Eilean Shona in 2014.

Is it funny that she and Richard both ended up owning islands? "I don't know how that happened," she laughs. "Mine is bigger than his." And better? "I didn't say better, they are very different. I love Necker Island [in the British Virgin Islands] and he loves Eilean Shona.

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"If as a family you can put time in together, you never regret it," adds Branson. "The thing about Eilean Shona is you always have an adventure there: kayaking to the beach, climbing over the mountain, having a campfire and cooking a sausage or mussels around it. It is a very special place."

One Hundred Summers by Vanessa Branson is published by Mensch, priced £20. Visit eileanshona.com