Alan Shadrake's career sparked into life when he uprooted his young family and relocated to Berlin shortly after the wall went up in 1961. Now the 76-year-old great-grandfather is enjoying a new lease of life after being imprisoned for writing a book that dared to criticise the judiciary in Singapore.

"I took two suitcases to Berlin, all the stuff I needed," he says when we meet at his daughter's house in Essex. "And I came back from Singapore with two suitcases and nothing of my life in between – no houses, no cars, no wives."

Shadrake has enjoyed a rip-roaring, itinerant writing career, his life shaped by the stories that he has pursued. "What have you done now, Dad?" his children laugh when he phones them from far-flung destinations. But Shadrake's family feared his determination to expose injustice in Singapore was an adventure too far. When he was found guilty of contempt of court last year for claims in his bestselling book, Once a Jolly Hangman, his youngest daughter emailed to ask: "Will they hang you Dad?" They did not but Shadrake has only recently been freed – and deported back to Britain – after five weeks in Changi prison.

Shadrake is still recovering from his imprisonment. "It was a pretty horrendous experience," he says. But he looks younger than his years. He began as a 15-year-old on the Hornchurch and Upminster News and worked his way up to the Daily Express in Manchester before moving to Berlin. There he filed stories for all the UK papers and wrote a book about the people-smuggling racket created by the wall. Later on, he moved to Los Angeles and Las Vegas, writing showbiz features for British tabloids.

After his third wife died, Shadrake was just about to go to Cuba when a journalist friend phoned and asked him to write some travel stories about Singapore. So he flew to south-east Asia. On his only night off he was sitting alone at a bar when he met a Singaporean-Chinese woman who was to become his fourth wife.

"The very morning I moved into her flat in Singapore I switched the TV on and there was an announcement about a British guy wanted for two murders who had run off to Australia," remembers Shadrake. That suspect was protected from extradition until it was guaranteed that he would not be put to death and yet Shadrake also spotted the case of Nguyen Van Tuong, a young Australian who was found guilty of heroin trafficking at Changi airport and was hanged.

"I saw it as double standards," he says. "I thought: find a hangman and interview him." The hangman he eventually tracked down changed his life. Shadrake was surprised to hit it off with Darshan Singh, Singapore's chief executioner for nearly 50 years who once executed 18 men in one day. "We're old codgers together," he says. "I used to talk to him about Manchester United. He loved it."

Shadrake envisioned a biography of Singh's life but as he spoke to more local people he became convinced the judicial system did not always deliver justice. And so Once a Jolly Hangman tells of a judiciary that embraced the death penalty for murder, drug trafficking and firearms offences. Singapore only recently started revealing its execution rates – killing six criminals in 2008 and five in 2009 – and Shadrake argued the death penalty disproportionately applied to the young and the poor, while high-ranking criminals, wealthy foreigners and well-connected drug lords escaped.

By the time the book was published, Shadrake had moved to Malaysia. He suspected his book would upset the Singaporean authorities but daringly ignored a warning from the British High Commission not to return to Singapore for his book launch. The evening before he flew in, he called his eldest daughter, Kim, and, with typical black humour, told her he was having his "last dinner". She joked: "Make sure it's a good one, it may be your last for a long time." Inside the launch party, 150 diplomats and activists toasted Shadrake's book. Outside, police circled in unmarked cars.

At 6.30am the next morning, Shadrake was woken in his hotel room by four police officers who ordered him to get dressed. He was interrogated for more than 10 hours a day for five days, a vent of cold air angled on to his neck. An officer opened a copy of his book and repeatedly asked him why he wrote certain sections. "It was a bit ridiculous. I said: 'This book is true,'" he remembers. Twice Shadrake was given the chance to publicly apologise before his trial and sentencing. He was also asked to promise not to impugn the integrity of Singapore's judiciary again. "I suddenly thought, what is this all about? I'm going to apologise and they'll let me go? Then I thought, I'm not going to apologise. They had fallen into their own trap. They had got me and they didn't know what to do. They had caused this global outcry. I was nobody and I'm nobody now but I've just become famous through their stupidity."

Facing 14 charges for "scandalising the judiciary" – in essence, writing a critical book that could undermine public faith in Singapore's judicial system – Shadrake became a global media story. He was found guilty of nine charges and sentenced to six weeks' imprisonment and a S$20,000 (£10,000) fine. Shadrake, who has heart problems and chronic back pain from a slipped disc, was forced to sleep on a hard floor with only a couple of thin blankets, just along from the prisioners on death row. He fell ill and was rushed to hospital – chained to the bed – and put on a drip. His care improved, he says, when he warned the authorities it would be a PR disaster for them if he was photographed arriving back in Britain in a wheelchair.

Before he went into jail, Shadrake made a vow: "When I come out, I'm going to continue to be a thorn in the side of the Singapore authorities for as long as I live." He is proud of what he considers his best work in his 50-year career and has "no regrets". After recuperating in Britain he will return to Malaysia, where his book has sold 10,000 copies – many to traders who have smuggled it into Singapore, where his book is not officially banned but is, mysteriously, unavailable in any shops. "The Singapore government scored an own-goal by persecuting Alan. His book has drawn the international spotlight to Singapore's use of the death penalty," said Lance Lattig, south-east Asia researcher at Amnesty International. "When it comes to drug trafficking, Singapore operates an institutionalised system of unfair trials with automatic presumption of guilt and a mandatory death penalty. Singapore doesn't recognise the universal right to free speech. Instead, the government tightens or loosens restrictions on free speech as it pleases."

Shadrake is in touch with human rights activists and Singaporean dissidents who have been energised by his book. He hopes that repression, injustice and the lack of separation between executive power and the judiciary in Singapore – criticisms that so inflamed the authorities – will come crumbling down just as surely as the Berlin Wall did as well. "It's going to be very slow. It might take 10 years but there are definitely changes going on. It's moving in the right direction now," he says. "I've had tremendous support in Singapore. I'm not a hero but I'm treated like one."