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Now his harrowing tale has been made into a critically acclaimed film, which went on general release yesterday. Eric is played by Colin Firth and Jeremy Irvine, who portrays him in his younger days as a Royal Signals officer. Patti is played by Nicole Kidman. Eric died before the film was completed, and so it has fallen to her to tell and retell her husband's story.

It took many years to understand the terrors buried deep within Eric's memory. "I was aware that he had been, as he put it, 'caught' in Singapore, but I'd been a wartime child so it didn't mean that much to me," says Patti, 76. Some 20 years younger than Eric, she is now growing frail herself, a slip of a woman who sinks into the chair in the London hotel where we meet. But in this, the sharing of her husband's experience, she steels herself. Her voice is measured, her eyes still dark with grief.

They were on honeymoon when she first realised Eric suffered from Post Traumatic Stress Disorder (PTSD). On their first night together, she awoke to find him thrashing and screaming in pain. "The nightmares were terrible," she says. "He had flashbacks, long silences, if I happened to say something that triggered a memory." Routine tasks such as going to the bank reminded him of being in an interrogation room. He would walk out of a restaurant if diners there were Japanese.

"It's very frightening and confusing if you don't understand what's going on," explains Patti. "He couldn't talk to me and it took a long time to finally get him to realise, and myself to realise, that he needed help."

This came in the form of an assessment at the former RAF hospital in Ely, Cambridgeshire, in the late Eighties, which saw Eric diagnosed with PTSD. But he still struggled to speak about his experience, and quit counselling after just one session. Soon afterwards, he spotted an article in The Daily Telegraph about a new charity, the Medical Foundation for the Care of Victims of Torture, and got in touch. So began two years of counselling, and, little by little, Patti – who accompanied Eric – began to learn the horrors of what he had been through.

Eric was 22 when he was captured by the Japanese. He was incarcerated at Kanchanaburi, western Thailand, and set to work on the railway intended to carry war supplies. Hundreds of thousands of prisoners of war toiled for 20 hours a day in 100F (38C) heat, malnourished and diseased. They had poor equipment, few rations and were beaten by their guards if they stepped out of line.

In August 1943, his captors discovered that Eric had built a radio so that he and his comrades could monitor the progress of the war. As punishment, he and six others were forced to stand in the heat for days. At night, they were stamped on and beaten unconscious with pickaxe handles. Two died. Eric survived, remembering only the crack of his own bones snapping and teeth breaking. As the ringleader, he was taken to another camp for interrogation. There, he was waterboarded and left to die in a cage the size of a coffin.

Of all his captors, the face of one lodged in Eric's mind like shrapnel. A young officer, Takashi Nagase, worked as an interpreter. "You will be killed whatever happens," Eric remembered him saying. Out of the blue, halfway through his counselling sessions in London, he received a letter from a friend. It contained a cutting from a Japanese newspaper, a review of Crosses and Tigers, Nagase's autobiography. His tormentor stared out at him from the page. Incensed, Eric got hold of a copy in English and started reading.

"It described Eric's tortures, and an experience that the author had had that made him feel he had been forgiven for his sins [in which Nagase felt he was encompassed in a golden light while standing in a war cemetery]," says Patti. "When Eric read the book, the curtains came down. I was incandescent. It is very unusual for me to be angry but to think this awful man had felt this while Eric was suffering…" She composes herself. "So, with Eric's permission – I never did anything without his permission, as it was his life – I wrote Mr Nagase a letter."

To her surprise, Nagase replied. "I had asked him how he could possibly think he was forgiven. I wanted to give him a real kick in the butt, metaphorically speaking," Patti explains. "His first letter back was very apologetic, touching really. We learnt that in atonement for the atrocities he had committed he had done a tremendous amount of charity work. We corresponded for many months before Eric felt ready to write back – and another two years before Eric wanted to meet him."

Patti accompanied Eric back to Thailand in 1993. The meeting with Nagase – then, like Eric, in his seventies – took place near the bridge over the River Kwai, the infamous stretch of Death Railway immortalised in a 1957 film. Footage from the day shows two grey-haired men tentatively shaking hands. "I must say something to you," Nagase pleads, bowing. "I am very sorry for what I have done. You must have suffered very much." Eric simply nods. "Thank you," he says. "Thank you."

He hadn't intended to be so forgiving. Up until the meeting, Patti admits, Eric planned to kill Nagase. "Eric meant to do him harm," she says, quietly. "He told me that he had fully intended to kill him. He would have garrotted him. My suspicions that he wasn't being quite honest about his reasons for wanting to meet this man were true. He wanted revenge. But then he realised this was another human being; he clicked into British officer mode. And, instead, he shook his hand."

As time passed, Eric was able to forgive Nagase, who died in 2011, and the pair became friends. "We met him a couple of times," smiles Patti, "him and his wife, Yoshiko. She and I became good friends. She can speak as much English as I can Japanese – virtually nil."

Eric's autobiography was published in 1995. He had started writing it on scraps of paper in Changi; the rest he finished during counselling. The final words in the book are those he uttered to Patti when the couple were standing in Kanchanaburi War Cemetery among the graves of Allied PoWs. "Sometimes, he said, the hating has to stop," says Patti. "And that sums up the man."

In the final year of his life, Eric went to watch a scene of The Railway Man being filmed near their home in Berwick-upon-Tweed. He was gravely ill with myasthenia gravis, a muscle-wasting condition, and his wheelchair was hoisted on to the town's walls to watch. "Eric didn't want to see the finished film," says Patti. "He was proud and touched. But he knew they would do such a good job that it would take him back to places in his memory that he wouldn't wish to go."

Patti, too, has suffered. Her only solace since Eric's death has been the hope that the film will raise awareness of PTSD, and its effects on veterans young and old. For the premiere, Patti had her husband's kilt, a rich green tartan, made into a dress. Today, she wears a poppy – and Eric's gold watch.

"He is with me always," she says. "This is the last thing I can do for him. He taught me what true forgiveness means. For that I owe him everything."

'The Railway Man' is in cinemas now