Arriving without fanfare, it was the ideal chance for the noble family to relax; villagers often not aware of their arrival or departure.
For it was the home to two of the Windsors' most beloved family members - Patricia Knatchbull, the 2nd Countess Mountbatten of Burma, or Lady Brabourne as she was better known locally, and John Knatchbull, 7th Baron Brabourne, often referred to as Lord Brabourne.
The property had been part of the sprawling estate which had been passed down through his family since 1486 and the reign of Henry VII.
The son of a former Ashford MP, he had built up an impressive career within the film and TV industry - being a top executive at Thames Television as well as producer on Academy Award-nominated movies Romeo and Juliet and A Passage to India.
Lady Brabourne had long been known to be one of the Queen's closest confidantes; the pair had been part of the 1st Buckingham Palace Girl Guides company - made up of children of royal household members - when both were youngsters in the 1930s.
She was the Queen's third cousin.
When the couple wed in 1946, the Queen - then Princess Elizabeth - and Princess Margaret were her bridesmaids. Joining them at the ceremony were King George VI and Queen Elizabeth. At the reception, the King wished the newlyweds "long life, every happiness, and the best of good luck".
They would need it.
While their Kent home held a special place in the heart of the royals - as well as a cherished family abode - it would also forever be tinged with sadness.
Because the hosts had been on the very fishing boat blown up by the IRA off the coast of Ireland in 1979 which sent shockwaves around the world.
The target was Lord Mountbatten - Lady Brabourne's father – and known as ‘Dickie’ to the Windsors and uncle to the Duke of Edinburgh.
Not only was the Admiral of the Fleet and the last Viceroy of India killed, but so too his grandson, Nicholas Knatchbull, one of Lady Brabourne's two 14-year-old twin sons. Other victims included her mother-in-law, the Dowager Lady Brabourne and Paul Maxwell, a 15-year-old who was helping crew the boat on the fateful day.
Both Lady Brabourne and John Knatchbull were badly injured in the blast - as was their other twin son, Timothy.
Yet the couple refused to let the tragedy - which saw her father's title pass to her - tarnish the rest of their life.
"John and Patricia were determined that life at Newhouse would continue to be as welcoming and as normal as possible following that brutal attack on the family," said Lord Crathorne, a long-time family friend.
"What was so inspiring was the way John and Patricia and indeed the whole family dealt with this tragedy. Their courage, lack of bitterness and generosity of spirit was remarkable. They were determined to overcome their injuries and put every ounce of energy into getting better. This was typical of two such exceptional human beings."
They had a lot to overcome.
The morning of August 29, 1979, was a sunny and warm one. The family were holidaying in one of their most treasured retreats. Lord Mountbatten and his family would often spend their summers in Classiebawn Castle - an imposing country retreat which was inherited by his late wife, Lady Mountbatten - perched on a hilltop in County Sligo on the north west coast of the Republic of Ireland - not far from the Northern Ireland border.
It was the last day of their break and Lord Mountbatten was keen to take to the waters again for some lobster and tuna fishing before they were due to return home. Joined by his family, they drove down to the nearby fishing village of Mullaghmore. Two Garda (Irish police) officers - despatched to shadow the family during his visit - kept a discreet eye out, parking on an overlooking cliff top after the family boarded the boat.
Lord Mountbatten had previously slimmed down his security attachment. He refused a bodyguard on the trip and insisted the Garda officers kept a watchful eye from a distance rather than up close. It is said a previous bodyguard joining him on his boat had got so sea-sick he opted not to invite them on-board again. This despite the obvious security concerns generated by 'The Troubles' - the 'low-level' war which had raged in Northern Ireland, and occasionally further afield, since the 1960s.
His boat, the Shadow V - a 29ft vessel described as "a cross between a fishing boat and a pleasure cruiser" - was something he was proud of. Kept under wraps for 11 months of the year in the castle's boathouse, it was always prepared for him for his annual visits.
The family were buoyed by both the good weather and the determination to make the most of the last day of their trip.
What no-one, apart from IRA high command and those sent to plant it, were to know was that a 50lb gelignite bomb had, under cover of darkness the previous night, been attached to the engine.
At 11.46am, they were just a few hundred yards into Donegal Bay when a radio signal was sent to the boat from a terrorist watching the vessel from the cliffs and the bomb detonated.
The size of the explosion shook the windows of those living on the mainland; a fireball erupted and a huge plume of smoke rose into the blue skies.
The vessel was completely destroyed - turned into splinters. The passengers thrown into the water as stunned villagers rushed to the scene.
Fortunately, the good weather meant other boats, some sailing nearby, were able to go to its aid swiftly. They found death and destruction.
Lord Mountbatten, 79, was killed instantly, along with the youngsters Nicholas Knatchbull and Paul Maxwell, who had been talking to him at the time of the blast.
Lady Brabourne, 55 at the time, recalled, many years later: "All I could remember of the explosion was seeing a ball of light, about the size of a tennis ball, radiating out from beneath my father's feet.
“Then I was in the water, turning over and over. I was certain I was going to drown.
"I have very vague memories, now and again, of floating among the wood and debris, being pulled into a small rubber dinghy before totally losing consciousness for days.”
She was left with serious injuries. Her face needed 120 stitches - including some in her eye-ball. "My IRA facelift," she would later call it.
The last words she remembered before the blast were those of her mother-in-law. "Isn’t it a beautiful day," she had said.
John Knatchbull was knocked unconscious and woke as he was being taken back to shore by one of the boats which had come to the rescue. Both his legs were broken. Looking at his wife, he feared she was dying as blood poured from her face. When treated in hospital, it was feared for a while he may lose a leg due to his injuries.
Timothy Knatchbull was pulled from the water - his face peppered with splinters generated by the blast. He was unable to see at first and his ears were damaged so struggled to hear. The Dowager Lady Brabourne, was recovered from the water conscious. She demanded the focus be turned on finding the children. She would die, not of her injuries but of shock, the following day. She was 83.
The body of Nicholas Knatchbull was the last to be recovered, more than an hour later.
Later writing a book on his experience, called From a Clear Blue Sky (his memory of how the life-changing attack occurred), Timothy Knatchbull would say on his website: "My family and I were relaxed and happy going out onto a flat calm sea in my grandfather’s fishing boat. My memories are intensely clear in short bursts.
"I remember climbing onto the roof of the cabin and talking to my grandfather who was steering. I have a distant memory of the sound of the explosion and of a very violent sensation and then nothing. Until a minute or two later lying in a boat and hearing anxious Irish voices talking at me.
"I felt intensely cold and knew that something was awfully wrong with me yet I didn’t quite know what it was. Another snatched memory of being put into the ambulance and seeing my father, and then later waking up in hospital from where my memories become continuous."
He was left with permanent damage to his right ear and hearing.
As his mother recovered in hospital, unable to speak, she wrote 'Nicky?' on a scrap of paper passed to a friend and was told he had not made it.
She later recalled: "It was overwhelming. I tried to cry but I could not even do that since the pain of the stitches around my eyes prevented me. It was as if a part of myself had died with my son."
The IRA – the Irish republican paramilitary organisation that wanted an end to British rule of Northern Ireland and were responsible for a pub bombing in Maidstone in 1975 and, later, the Deal Barracks atrocity in 1989 - issued a statement later that evening. It said: " The IRA claim responsibility for the execution of Lord Louis Mountbatten. This operation is one of the discriminate ways we can bring to the attention of the English people the continuing occupation of our country."
Ironically, Lord Mountbatten was one of the few senior figures in the British establishment who supported an autonomous Ireland.
Timothy and his parents were both too ill to attend Nicholas' funeral at the family plot in Mersham.
Only one man has been convicted of the attack - Thomas McMahon. He was released in 1998 as part of the Good Friday Agreement which finally brought an end to the conflict. Now in his 70s, he lives with his wife in Ireland.
As viewers of Netflix’s hugely popular The Crown will be aware, Lord Mountbatten had been particularly close to Prince Charles and the two corresponded regularly.
His death and those of the others on the boat that fateful day created headlines around the world.
Lady Brabourne later said: "“I was so overwhelmed with grief for Nicky, who was just on the threshold of his life, that I began to feel guilty that I was not able to grieve for my father, whom I really adored, in the same way. But the world was mourning him, and there was comfort in that.”
But Lord and Lady Brabourne refused to allow the tragedy to dominate their lives - despite their profound and devastating loss (Lady Brabourne said she wept every day for a year after the explosion). They refused to carry the cancer of bitterness with them.
Their son, Timothy Knatchbull - one of the couple's eight children – would explain: "My return visits to Ireland equipped me with a greater understanding of the political situation in which I had found myself in 1979 and an equally greater understanding of my own feelings. I gained a firm base for the forgiveness which had crept over me in the intervening years. My parents were instrumental in this and I was guided by their ongoing love for the Irish and their complete lack of bitterness."
And he added the Windsors had helped the family greatly in the aftermath of the tragedy.
He said: "Our families have been historically entwined for nearly two centuries in lineage and friendship. The Duke of Edinburgh is my mother’s first cousin, the Queen is her third cousin and they have both been close to my mother since their childhood.
"The royal family were kept informed of our progress and on our return to England they were among a number of close family friends who cared for me while my parents remained in hospital. The Queen and her family are a supportive and loving set of people who were able to do a tremendous amount of good to me personally and also to my wider family, helping us to get back on our feet after the most difficult time any of us had been through."
Prince Charles is godfather to both her and his late identical twin brother.
In 2012, the IRA's former chief of staff, Martin McGuinness shook the hands of the Queen in a symbolic meeting in Belfast. He had played a key role as Sinn Féin's chief negotiator during the peace process of 1998 - Sinn Féin was for many years seen as the political arm of the IRA. He said of the meeting: "I liked her courage in agreeing to meet with me; I liked the engagements that I’ve had with her. There’s nothing I have seen in my engagements with her that this is someone I should dislike – I like her.”
In 2015, Prince Charles - who had been on a fishing trip in Iceland when he was informed of the explosion - and the Duchess of Cornwall joined Timothy Knatchbull in a visit to Mullaghmore.
Added Timothy Knatchbull: "The bomb left me with a legacy of mental and emotional wounds which refused to go away. By returning to Ireland and piecing together the story my symptoms started to fade and I found a sense of inner peace that I had lost the day my twin was killed. I reached a point where I could accept, understand and move forward in a way I had never done before, welcoming a whole range of new possibilities in life. And I was finally able to say goodbye to Nicholas."
Lord Knatchbull died in 2005 at the family home in Mersham. He was 80.
Lady Brabourne, a popular local figure, passed away in 2017 at the home she so cherished. She was 93. Both died surrounded by their family. The Queen, Duke of Edinburgh and Prince of Wales were among the mourners at her funeral.
Last month, many of the belongings of Newhouse were sold by auction house Sotheby's - the sale exceeded expectations generating more than £5.6million. The Countess had instructed her family to do so before her death.
Lord Crathorne, writing in the brochure which previewed the sale recalled his time at the home: "Guests at Newhouse were cosseted and made very comfortable. Evenings were very relaxed with guests enjoying drinks and perusing The Guardian, The Times, The Mirror or indeed the Kent Messenger."
Timothy Knatchbull is now the last survivor of that terrible day in 1979. Married with five children, he describes himself today as a "media executive and entrepreneur". He is now 56.
The death of the Duke of Edinburgh marks the passing of yet another era defining character - an era in which Lord and Lady Brabourne played a significant role.