Hidden in remote Snowdonia, a 17th-century country retreat has housed a treasure trove of paintings, historic furniture and lost letters that shed light on one of the great romances of the Sixties. Philippa Stockley meets Jasset, 7th Baron Harlech to hear more

It takes longer to get to Glyn Cywarch than it does to New York. At Tegwyn, a single-track station in remote north Wales, you must hail the little train to make it stop for you, much as you would a taxi.

Then, after a few minutes' journey through the depths of Snowdonia National Park, you reach an early 17th-century country house: this is Glyn. On the death of his father Francis a year ago, it was inherited by 30-year-old Jasset Ormsby Gore, now the 7th Baron Harlech. And he already has big plans for the place.

With his open, appealing smile and attractive self-deprecation – "I look like a builder," he says – Jasset is going to restore the old house, ready for a new life. With the help of his mother, Amanda, Lady Harlech, and his younger sister, model and actress Tallulah, he plans to turn it into "a sort of country guest house". Like most ancient houses, it needs work. "I'm very blessed and very lucky to have this connection to this part of the world," he says. "It's going to be a long road ahead with some tough decisions, but I'm very optimistic about the future of the estate." One of those tough decisions was to sell much of the fine 17th-century oak furniture, paintings, and other items from the house – offered at auction by Bonhams in London this March – to fund the project, which Jasset estimates will take at least a year to complete.

Today, the original gatehouse (with later Victorian wings) leads to clovered lawns and lichened steps up to the thick-walled, three-storey Grade II*-listed stone building, made of two conjoined houses, with an old slate roof and attractive dormered attics. Behind, the walled garden, with a bold pink climbing rose, holds pleached trees; lower down are outbuildings that include an ancient watermill and timber yard, reminders of a working estate that, according to Jasset, once had a staff of 50. Three thousand acres of now tenanted sheep pasture and farmland lie beyond.

Built by William Wynn in 1616, the house and estate passed by marriage to the Owens of Clenenney and Brogyntyn. Palladian Brogyntyn Hall in Shropshire (sold in 2001) became the main seat, while Glyn Cywarch was the agent's house. But in the 19th century, Glyn began to be used for summer visits. In 1876, the title of Baron Harlech was given to the Conservative politician John Ormsby Gore, a former groom-in-waiting of Queen Victoria. Around that time, improvements were made to the house.

The house is ravishing, with stone-flagged floors, glorious oak panelling, great fireplaces bearing Wynn's arms above the mantels, and deep-set windows. Yes, the wallpaper peels dramatically in the attics, but the property has happy proportions and lovely diffused light. When I visited, after the late Lord Harlech died, it was furnished with a profusion of 17th-century oak: coffers, a gigantic bulbous-footed refectory table, a great dresser in the kitchen, and a pair of rare 17th-century three-tier buffets, one used casually in the back hall. All this was tempered with fine paintings and oddities such as a shining black hairball found in the stomach of a 19th-century cow, and coats for hunting and from old uniforms draped on the backs of chairs. There was fine furniture everywhere, most of it solid and well-loved. In the kitchen, numerous bright copper pans proudly marked with the Harlech 'H' dangled gleaming from a rack, along with other kitchenalia, including cutting blocks and a pestle the size of a small knobkerrie. "My father was an amazing cook," Jasset says. "He could have been a chef. Even if you had to wait four hours for breakfast, it would be the best breakfast you ever had; and at dinner parties 16 people would sit spellbound, hanging on his tales of derring-do. He was a bon vivant and a raconteur."

Among more than 400 lots from the house, highlights include 12 carved George II mahogany side chairs in needlepoint upholstery with a William Morris feel, their fat shell-cabriole legs resting on lion paws. There are also seven George III gilded armchairs in Adam style, upholstered with one of Aesop's fables in gros- and petit-point – the eighth is in a museum.

Daniel Quigley's 1795 painting The Godolphin Arabian is compelling, both for its lack of anatomical reality – Quigley had to imagine the famous thoroughbred, brought 65 years earlier to the stables of the 2nd Earl of Godolphin – and for the tale it tells. This lightning-legged stallion arrived in 1730 via Syria and France. He was believed to have been a gift to Louis XV, who, unimpressed, used him as a carthorse. Today, the stallion's descendants still astonish racegoers.

Looking on is a magical 16th-century portrait of Ellin Maurice, wife of John Owen. This painting, with its exquisitely done farthingale, gleaming ropes of pearls and splendid ruff, has just been attributed to Marcus Gheeraerts the Younger and it strongly recalls his so-called 'Ditchley Portrait' of Elizabeth I from 1592.

Although he owned many beautiful things, from dressers to decoy ducks, the 6th Baron, Francis Harlech, was a hard-working peer, concerned about farming. According to his son, he was an excellent engineer too. He ran a small haulage company from Glyn, and sometimes drove his own lorries to Westminster to sit in the Lords. In the coach-house to one side of the gatehouse, under the billiard room and library, a curvaceous racing-green Lagonda Rapier 1 Sports Tourer, last driven – its tax disc hints – in 1984, mutely begs to be let rip again. Beside it gleams a 1925 black Douglas motorbike that Jasset says is "in pristine condition: you could start it and it would go".

Jasset remembers the ancient boiler his father used to strip down, clean, and rebuild, which "sounded like a Rolls-Royce jet engine when it fired".

Jasset and his younger sister Tallulah were brought up in Shropshire, but they stayed at the Welsh house for Christmas and during the summer holidays. He describes the area's "intoxicating atmosphere", and talks about its romance: "Our father said that north Wales was the most beautiful place in the world, and insisted we spend our holidays there."

The new Lord Harlech says that the farmland is profitable, but that the house needs restoring. He is upbeat and clear-sighted about the changes that must be made. Before him, his father dealt with swingeing death duties at the death of his father, David Ormsby Gore, the 5th Baron Harlech, by selling some paintings. Jasset speaks of that grandfather with respect and pride, recalling his roles as a former ambassador to America, a close friend and adviser to President Kennedy. David Ormsby Gore also set up HTV (it stood for Harlech TV) and served in the Second World War. In 1939, aged 21, as a member of the Berkshire Yeomanry, he had operated behind enemy lines with the Phantom reconnaissance unit. In Glyn, the 5th Baron's leather-topped desk not only held signed Kennedy photographs, but also a heavy JFK memorial silver cigarette box, inscribed to him, and his engraved despatch box – these very special items are also to be offered by Bonhams.

Now Jasset intends to restore Glyn "for many people to enjoy; to put it back to flagstones, panelling and limewash, as it should be", and to restore the slate roof.

He has already built a successful career in film, working his way up in a production company from teaboy: "I may go back to it, but for now I'm 110 per cent focused on turning this into a viable business. The Bonhams sale will make this a reality."

He is in an enviable position in the sense that he can call on the help of his sister, Tallulah and his mother, Amanda, Lady Harlech. Well known as a creative consultant, Amanda Harlech has famously worked with John Galliano and Karl Lagerfeld. Jasset fondly remembers a 20-page Bruce Weber shoot with his mother for Vogue, done at the house one rainy day when he was little. He describes how he and Tallulah were allowed to be in it, and he wore one of the uniforms.

As he talks of his mother and sister, his face lights up: "Mum and my sister have a wonderful aesthetic. It's so important to have the right bed linen and the right colours. It's my job to do the nuts and bolts – the roofing and so on – and to create a canvas for them. They're so excited... I have to tell them, 'Guys! You'll have your chance to make it even more beautiful.'

"It will be fantastic when it's finished. I'm optimistic and enthusiastic that we'll get it done, and I can't wait to open the door."

Philippa Stockley reviews for the Sunday Telegraph and Country Life, and writes for the Evening Standard among other publications.