For the crew of the small wooden boat heading to the fishing grounds off the coast of Guatemala, it must have been a sickening discovery. Floating in the Caribbean waters less than 200 yards from the shore were the bloated corpses of two young Westerners, a man and a woman. They had been bound, tortured and weighed down with heavy engine parts. Their identities were unknown.

It was July 8, 1978, and 5,000 miles away in Manchester, we knew nothing except our deepening concern about the fate of my brother Christopher Farmer, a 25-year-old newly qualified doctor, and his lawyer girlfriend Peta Frampton, 24.

They had set off together at the beginning of December 1977 to fulfil their dream to see the world. For seven months they had kept their promise to keep in touch as they travelled through Australia and the Pacific islands to Los Angeles and then through Mexico to Belize. In the age before the internet, Chris made regular phone calls home and Peta sent wonderfully detailed letters to her mother, who lived across the road from us.

Innocent victims: The last-known picture of Peta Frampton and Chris Farmer together, taken on the day before they set off on their globe-trotting adventure

I was 17, and still at school. Then came silence – and the first steps in a deeply painful quest for the truth that would eventually span two continents and the best part of the next four decades as we fought our way to justice.

The last contact we had came in the form of a letter from Peta, dated June 28, 1978, which explained how they had agreed to sail down the Caribbean coastline from Belize to Honduras with an American sailor called Silas Duane Boston.

With his weather-beaten face, she wrote, Boston was an engaging character with a devil-may-care attitude, small, piercing blue eyes and a large tattoo of a mermaid on his left forearm. What he lacked in conventional good looks, he made up for in megawatt charm. It seemed he had been married no fewer than five times and had driven to Belize from his home in Sacramento, California, with his two sons, 13-year-old Vince and Russell, 12.

There he had bought a 32ft wooden sailing boat, the Justin B, which he used to ferry tourists to the palm-fringed Belizean islands for what he described as a ‘Robinson Crusoe’ experience, mooring at white sandy beaches and spearing fish for supper. Now, explained the letter, he had offered Chris and Peta the chance to sail 150 miles with him and his boys down the coast to Honduras for $500 (about £1,500 in today’s money). For the young couple, it seemed too good an opportunity to miss.

As the summer wore on we heard nothing else. That last letter was all we had to go on, and my father was growing increasingly desperate for information. On September 22, 1977, he wrote to the harbour master in Belize City asking for records of the Justin B. It was weeks later before he finally got a reply, and when it came it was worrying. It said that Christopher and Peta had indeed been on the crew list when it left port; yet there was no record of them on board when the Justin B next docked.

Another letter from the Honduran authorities revealed that the pair had bought visas for Honduras, but that they had never entered the country.

Back in Manchester, our efforts intensified. At the behest of my parents and Peta’s family, the Foreign Office alerted the British vice- consul in Guatemala, which borders Belize to the north and Honduras to the south.

The best chance of progress still lay with Boston himself and, in mid-October, the Foreign Office called to say that the man and his two sons had been located in California.

At first he was evasive. When an official from the British Consulate in San Francisco spoke with him on the phone in October, he insisted that he had dropped Chris and Peta back in Guatemala.

His vague responses raised more questions than they answered, and a few days later my parents, now deeply suspicious, went to Greater Manchester Police (GMP).

At the same time, the British Consul was sufficiently concerned that he sent an official to interview Boston in person on December 3, 1977. He appeared ‘calm and relaxed’, said the official.

However, when questioned about Chris and Peta, ‘Boston sat straight up in the chair, his eyes widened and his breathing became heavy’.

Chris with Russell, left, and Vince Boston in Belize in a snap taken about a week before his murder and kept by Russell

‘Following this, he slumped back in the chair, placing his face into his right hand, and in a softly spoken voice he said that he thought Chris and Peta would be back home by now,’ the official reported.

With no concrete evidence, the official investigations hit the buffers. In an attempt to keep the case alive, Dad, who worked for the BBC, placed articles in the British press and appealed for information in the Belize Times newspaper.

Among those who got in touch was American Dr Tom Lane, who in turn enlisted a Belizean friend, Alphonso de Pena, to act as a private investigator – and it was de Pena who made the first significant breakthrough. At the end of January 1979, de Pena was told by a Catholic priest in Livingston, Guatemala, just south of the Belize border, that local fishermen had made a terrible discovery: they had found the bodies of a young European couple in the water the previous July. A navy diver had cut the bloated corpses free from ropes anchoring them to engine parts on the seabed 200 yards from shore. Buried in unmarked graves, the dead couple were never identified.

The news hit our families like a hammer blow. We all knew it must be them. How many other missing young Europeans could there be in such a remote area?

We arranged for their dental records to be flown to Guatemala and paid £450 for an exhumation that would prove the matter.

Nine weeks later, we finally learned the truth about Chris and Peta. They had been tortured, tied and beaten before drowning.

Our belief that Boston was responsible was stronger than ever when we learned through Interpol that Boston’s third wife, Mary Lou (the mother of his two boys), had disappeared in September 1968.

Manchester Police asked detectives in Sacramento to interview Boston and his sons in May 1979, but it never happened. He had taken his sons and disappeared.

We lived with our loss for the next 37 years. While the passage of time means you think about it less, the sense of pain never diminishes. It was not until 2015, after my father’s death, that the lightbulb moment came. Mum said wistfully: ‘I wonder what Chris would have looked like. He’d have been 62 now. He’s forever young in our eyes, isn’t he?’

It suddenly struck me that we now live in a different age. We have the worldwide web as a source of almost limitless information, rather than the painfully slow exchange of letters of the late 1970s. I began to scour the internet, driven by the belief that if I looked hard enough, I would get to Boston – and the truth.

I didn’t have to dig far to find his eldest son Vince on Facebook. He was now a 50-year-old aviation electrician living in Arizona and a vocal opponent of America’s free-and-easy gun laws.

He wrote on his page: ‘My mother was killed at 23 with a gun.’ Remembering what the police had told us, my eyes popped out on stalks. His brother, Russell, I found, was a 49-year-old illustrator living near Laguna Beach, California. My head was spinning, we always knew the brothers were the two people we desperately needed to talk to and I sent them private messages urging them to tell me what had happened on the boat.

Monster: Silas Duane Boston in Sacramento in 2012, 34 years after the murders

By now, I was like a dog with a bone and soon tracked down Boston himself. His Facebook picture was of a grizzled 74-year-old with a white beard wearing a T-shirt under a denim shirt, a baseball cap and sunglasses. To me, he looked like trailer trash – or a serial killer from a horror movie. Though I instantly hated him, my overwhelming feeling was relief he was still alive to face justice.

I decided to contact the cold case unit at GMP. Our expectations of them reopening the case were low. But we hadn’t banked on an extraordinary set of coincidences.

The first came soon after discovering the case files had not been lost. For some reason, the long-retired detective in charge had kept a complete copy. We were in business.

The new investigating officer, Detective Constable Michaela Clinch, contacted Interpol, where she was put in touch with an astonished Detective Amy Crosby at Sacramento Police.

Quite coincidentally, she had raised the case with Interpol at almost the same time because of Amy’s reinvestigation of the disappearance of Vince and Russell’s mother in 1968.

Vince gave a statement to Amy on October 13, 2015, 11 days after I had sent him a Facebook message and the day after we had been to GMP.

He told her that it was an open family secret that his father had killed his mother, but no one knew where he had buried her.

Vince then told Amy he had witnessed the cold-blooded murders of Chris and Peta on board the boat.

No hope of escape: An engine part identical to the ones used to weigh down Chris and Peta

It would later emerge that he had repeatedly tried to alert the authorities to the killings, but with no success. Vince had joined the US Navy in 1982 at the age of 16, and his first act on escaping Silas Duane Boston’s evil grip was to tell police in London what he had seen four years earlier. He gave Chris and Peta’s full names, but was told there was no file on the case.

Further attempts by Vince and his brother Russell to get the case taken seriously on both sides of the Atlantic fell on deaf ears.

It was our visit to Greater Manchester Police in October 2015 that proved the game-changer. The case was no longer cold but suddenly very much active – particularly when Russell corroborated Vince’s account and produced crucial photographs showing Chris and Peta aboard the Justin B.

We were called to a meeting with GMP’s force review officer, Martin Bottomley, who had Russell and Vince’s statements in front of him. He quietly asked: ‘How much would you like to know?’ Without hesitation, Mum replied: ‘Everything.’

It seemed to us that Chris and Peta’s ghosts were talking from beyond the grave when Martin started reading.

As the story unfolded, we realised the stomach-churning terror they must have felt and just how unlucky they had been to chance upon a psychopath on a remote Central American island. It transpired Boston was in Belize because he had skipped bail on a charge of statutory rape of a minor in Sacramento. The boys described the purchase of the Justin B and how their father would become violent after drinking cheap local rum.

Chris and Peta were on the boat, they said, when a drunken Boston started beating up Russell. When Chris intervened, Boston tried to swing a punch at Chris and fell humiliatingly into the sea instead.

According to Vince, that was when Boston began plotting their murder. The following evening, he told Chris to pull up the anchor, crept up behind him and repeatedly bludgeoned him over the head. He then attempted to stab Chris in the chest with a fillet knife until the blade broke and Chris cried out: ‘I give up!’

The following morning, Boston told Chris and Peta he was going to drop them near Livingston, but would tie their hands and strip them naked to stop them reporting him to the police before he escaped. Over the next 36 hours, Boston taunted them before trussing them, putting plastic bags over their heads, tying them to blocks of metal and throwing them overboard fully conscious. They never had a hope.

When Martin finished reading, a shocked hush descended on the room. The fog that had enveloped their disappearance since 1978 had cleared and the full extent of Boston’s evil was revealed.

Even after all this time, the details were hard to bear. We left the room shell-shocked, but grateful for finally knowing the truth.

After a lifetime spent on the run, Silas Duane Boston’s loathsome past finally caught up with him at the age of 75

After a lifetime spent on the run, Silas Duane Boston’s loathsome past finally caught up with him at the age of 75. On December 1, 2016, he was charged with the murders of Chris and Peta.

It is unlikely they were his only victims. Today, Sacramento Police have two large files, totalling 2,000 pages, detailing five decades of crimes Boston is suspected of – including numerous murders. But after years of alcohol abuse, his health was already failing and Boston died in custody on April 24, 2017, three weeks before Mum and I were due to fly out for a pre-trial hearing.

Boston was defiant to his last gasp. The prison guards described him as ‘controlling with an evil glare right to the very end’.