The last time Hilary Holland heard from her brother was in June 1978, when he was about to set sail from Malaysia to Thailand.

When the letters stopped she presumed he was busy travelling. It wasn't until 18 months later that she found out the devastating truth - that he had been captured, tortured and murdered by Pol Pot's Khmer Rouge regime after straying into Cambodian waters.

John Dewhirst, who was brought up in Cumbria, is the only British victim of the Cambodian genocide in the late 1970s.

Hilary, of Castle Carrock, still doesn't know exactly how he died. What she knows it was undoubtedly horrific, after weeks of torture.

It has taken 35 years for her even to be able to speak about what happened. Now, as two senior members of the Khmer Rouge go on trial, she has taken the difficult decision to share his story, so that nobody forgets the atrocities committed in Cambodia - where 25 per cent of the population died under Pol Pot's regime.

"It's not as high profile but it's no less horrific than what happened with Hitler and the Nazis. The world will never forget that, but still a lot of people still don't really know who Pol Pot was," she said.

Originally from Newcastle, Hilary and John moved to Cumbria as children and grew up in the village of Renwick, in the Eden Valley.

John later left to go travelling, originally moving to Japan where he taught English as a foreign language. "We wrote to each other fairly regularly. Usually about once a month. The last letter I got from him was in June 1978," said Hilary, now 61.

In it, John - who she said always had a sense of adventure - set out his plans, then she didn't hear any more.

"I didn't start to worry for quite a few months. What was going on in Cambodia wasn't really known at the time, otherwise I might have made an early assumption. As it was, I wasn't desperately worried. I thought he was travelling and hadn't been able to write. I became more and more worried as the months went by," she said.

Hilary, a retired solicitor with Cartmell Shepherd, started to write to people to find out if anyone knew where he was.

In the days before email everything had to by sent via post, and it could take weeks or months to get a reply.

"It got to January 1980 - a good 18 months after I'd last heard from him - and I got a call off the Foreign Office. It was just a message really. That we think your brother has probably been captured by the Pol Pot regime and probably taken to Tuol Sleng and murdered. It was such shocking news she couldn't really take it in," she said.

It turned out that for some reason, possibly a storm, the boat he had been on had strayed into Cambodian waters

"He was with two other people - a New Zealander and a Canadian. The boat belonged to them. He was a paying passenger going from Malaysia towards Thailand," explained Hilary.

"A gun boat came and attacked them. The Canadian was killed outright. My brother and the New Zealander were captured."

They were taken to Tuol Sleng prison, also known as S-21.

After the call from the Foreign Office, Hilary heard from journalist who was one of the first to reveal the atrocities from within the high profile prison. He gave her an insight into what had been happening.

"The term they used was 'smashed' - meaning torture and murder. It was so distressing. It's so difficult trying to come to terms with a death in these circumstances. It's not like he died in a freak accident or had an illness and died. He was so horrifically treated for weeks, before being murdered in some brutal way. It's the torture and brutal death. Knowing that happened, the suffering - it just won't go away.

"I don't know how he was killed but the methods of torture are generally known and it's absolutely horrific," she said.

Hilary has heard an account from one of the guards, that he was taken into the streets and set on fire. She doesn't know if it's true. But she knows that however he died, he would have suffered.

"I keep the whole thing buried away because I can't deal with it. Thinking about it makes it more horrific. But I have decided to speak now because I think the world needs to know. It mustn't be allowed to forget. Even in Cambodia the younger generation don't really know about what happened. It isn't taught in schools," she said.

During Pol Pot's regime about 15,000 people went through Tuol Sleng prison, but only about half a dozen survived.

John was one of only seven non-Cambodians to end up there, and the only Briton. "The aim was to produce a confession, along the lines that they were CIA spies," said Hilary.

"I've got a copy of my brother's confession. It was very long, very detailed, saying that all his family were CIA spies. If it wasn't so horrendous it would be a joke."

She has read others that are along the same lines. From what she has been able to find out, the confessions had to be approved by prison chief Brother Duch - real name Kang Kech Ieu.

He would send prisoners back time and time again to be tortured until they produced the confession he wanted. Once he was happy with it, they would be murdered.

Duch, now in his 80s, has already been sentenced to life in prison.

Two other Khmer Rouge leaders are currently on trial - Pol Pot's deputy Nuon Chea and head of state Khieu Samphan - at the Extraordinary Chambers in the Courts of Cambodia (ECCC) and Duch has been among those giving evidence.

But Hilary says there is another man who still evades punishment, living in freedom somewhere near the border with Thailand.

"This person, whose name I don't know, was in charge of the non-Cambodian prisoners. He's probably the only one that really knows what happened to my brother but he's never been tried," she said.

It has been suggested she write to him, but she is still undecided.

The prison is now the Tuol Sleng Genocide Museum and this year Hilary finally took the brave step of visiting it.

"It's taken me 35 years to pluck up the courage to go there.

I've been retired two years so I felt it was time to make myself face up to it. I think it was really important to go. I'm much more able to talk about it now. Before that I was pretty incapable.

"But it's reminded me of the importance of what happened. In this country I'm the sole voice. Nobody else can tell the story. I don't feel it's about me. It's about making sure there's an understanding of what happened in that country. It's of massive international importance both from a historical point of view but also the social relevance of something like this potentially happening again," she said.

Because John was the only Brit affected, she hasn't even been able to share her pain with others. Then she met the brother of one of the other non-Cambodian victims. "I found out that he felt exactly the same. It was good in a way to share that experience," she said.

"But there must be a whole country of people in Cambodia who must feel like that. It's so far removed from anything you can imagine. It's horrendous now as it was 35 years ago."

Hilary speaks of a "debilitating grief" that still remains with her, describing it as feeling like they "took the humanity, the soul" from her brother by subjecting him to such terror.

She added she can never forgive such inhuman acts, as she feels that all of the happy memories of her brother have been taken from her - tainted by the knowledge of what happened to him in Cambodia.

She does not demand retribution, but hopes the trials will give some psychological comfort to the Cambodian people and remind the world what happened there.