Pictured: Penny Farmer, now 57, of Manchester

Penny Farmer was just a teenager when her brother and his girlfriend were killed during their a round-the-world trip.

Now 57, Penny, of Oxfordshire, says her family was denied justice after their murderer Silas Duane Boston, died in prison shortly after being charged.

She said: 'I was gutted, absolutely gutted. I thought we were going to see justice.

'I was ready to stand up in court and tell them what a monster he was. But he wouldn't even let us have that. It took quite a while to get over that. The game was up for him.

'I wouldn't like to say suicide but he did it for a reason and that was because he knew the game was up. It was really gutting.'

Penny's brother Christopher Farmer, 25, a newly-qualified doctor, and his sweetheart Peta Frampton, 24, a lawyer, had met Boston during their journey from Belize to Mexico.

He had offered to help them get to their desired destination by taking them on as crew members on his 32ft wooden sailing boat, the Justin B.

But by the first week of July, 1978, seven months after the couple had left Manchester and starter their vacation, the two were tortured by Boston.

They were beaten, bound and thrown into the sea, weighted down by heavy machine parts tied to them.

After escaping police for 38 years, Sacramento Police finally tracked down Boston in December, 2016, and he was charged with the murders of Christopher and Peta.

Penny and her family pleaded with the court to hasten proceedings, as her mother Audrey, now 93, wanted to see justice done before she died.

The family was even willing to forego an entitlement to seek the death penalty upon conviction in an effort to speed matters up.

Peta Frampton and Christopher Farmer the day before they set off in December 1977

But still Boston managed to conclude matters on his own terms. With deteriorating health because of years of alcohol abuse and with just three weeks before Penny and Audrew were due to attend a pre-trial hearing, Boston died in jail, aged 76.

He had been on dialysis but he is said to have ordered medics to withdraw treatment.

Within hours, Penny and Audrey, who had been preparing to travel to the US, were told the news. There would be no day in court for them. They would not have the satisfaction of looking him in the eye in the dock.

Despite finally getting to the truth, Penny despairs that Boston has escaped justice.

'I'm sorry but four months in prison does not equate to justice in my book', she said.

Boston was finally discovered and charged for murder after Penny decided to follow up her father’s detective work with the help of Facebook.

Christopher's father Charles Farmer, a BBC journalist, made his own enquiries to find out where the pair were after communication from the two had stopped.

He appealed for help in the Belize Times to no avail. Following this, he hired a local man, Alphonso de Pena, to act as a private investigator.

By January 1979, Mr de Pena learned through a local priest living just over the border in Guatemala that the unidentified bodies of a young European couple had been pulled from the water 200 metres from the shore the previous year.

After the private investigator's breakthrough, the bodies were exhumed and dental records were flown across the Atlantic to confirm it was Christopher, 25, and Peta, 24.

It was devastating news for the families back home in Chorlton.

Charles and diplomats at the Foreign Office continued to make enquiries, tracking down Boston, who - it would later emerge - was on the run in central America, wanted on a charge of statutory rape against a minor back home in Sacramento, California.

The British consulate and Charles both managed to speak to Boston by phone.

He was evasive and unconvincing, telling the consulate his two passengers had disembarked because his boat had required repairs.

To Charles, he said: 'Let me know if you hear anything about them', claiming he didn't know what had happened to the couple.

Duane Boston (pictured) was charged with murdering Christopher and Peta

After that, Boston seemed to disappear off the face of the earth. Charles Farmer died in 2013, at the age of 91, without ever getting to the truth.

Fast forward nearly 40 years and Penny, after a nagging sense of injustice gnawed away at her, decided to conduct her own search.

She found Boston and his two sons, both aged 13 and 12 when on the vessel, who admitted that their father was the killer.

The two men said they had witnessed their father tie up the couple and toss them into the sea, apparently furious that Christopher had chided him for bullying his younger son aboard.

Greater Manchester Police contacted Interpol and Sacramento Police, who quite by chance had reopened an investigation into the disappearance of Vince and Russell's mother Mary Lou in 1968.

Her disappearance was one of a dozens of apparent murders and rapes which police in California had re-examined.

Today Sacramento Police is said to have two large files totalling 2,000 pages which implicate Boston in crimes going back 50 years, including the death of Mary Lou and other murders.

Now, Penny has written a book Dead In The Water about the crime and her efforts to get to that truth, which had remained secret for almost 40 years.

Penny explained: 'First of all I wanted it to be a lasting memorial to Chris and Peta who were two great people. They were so loved. There's not a day goes by we don't think about them. Secondly, you can't deny it's a truly remarkable story.

'It's the most incredible story which I wanted to be told accurately and truthfully and my family and I are part of that story. Why wouldn't I write it? It's my family's story to tell.'