'I was so close to my grandmother, partly because I never felt that my father liked me. He was so disappointed that I was not a boy. He hated me so much,' a tearful Sun says. 'My grandmother was kind. She would try to go to the temple and my mother would be so rude. I thought my grandmother was strange, too. I was always on my father's side. My whole family were. I think my grandmother was so lonely and that now makes me feel awful.'

Sun, 39, recreated the journey of Xuanzang, the monk who travelled to India hoping to find scriptures in the home of Buddhism that would reconcile strands of the religion that had developed in China. He returned 18 years later with 600 books of sutras, seven statues and enough information from India and the Silk Road to write the Record Of The Western Region. Xuanzang's religious and secular writing was crucial to China's golden period, the Tang dynasty, whose emperors took advantage of the Silk Road to learn about the outside world and encourage envoys from other countries to visit China.

Sun contrasts the generosity of the Tang and the insularity of the revolution. Where Xuanzang helped create one of history's greatest empires by leaving China, Sun's journey was about discovering her homeland and finding a place for herself as an overseas Chinese whose father's death as a disillusioned cadre cut her last tie with the Communist Party. 'This book is really the culmination of me trying to understand my culture,' she says.

At her father's funeral in 1997, Sun noticed his elderly comrades had returned to some of the 10,000 temples rebuilt across China over the past decade. 'Seeing that my father's friends were going to temples made me think about the futility of the revolution and the things we attacked,' she says.

It occurred to her, she says, that China and Buddhism were inexorably linked and that if the religion could survive the revolution its intellectual contribution to the country and its place in the national psyche must be immovable. 'Understanding the true Buddhism will help Chinese people a lot, even if they don't become Buddhists,' she says.

By tracking the re-emergence of Buddhism in China, she hoped to learn more about the nation's history and about her father, who supported Mao Zedong's bid to strangle the religion. 'His generation suffered so much. He didn't care so much for his family - he couldn't. He gave freely his life for the Cultural Revolution. Up to the end of the revolution he was so busy trying to make it work.

'He could not understand why it didn't work out; why, even when I went to university, we were still living on rations of food, oil, water, everything.

'My father constantly had to send money, medicine, sweets and clothes to my relatives in the countryside. A lot of western China is still like that. He just thought, 'How could it all have gone so horribly wrong?' I think he really lost heart,' Sun says.

'When the reforms came, his generation was seen as failing to achieve the utopia. They were pushed aside - rightly so. I think he really never recovered from that and I have so much sympathy for him. His whole generation tried to do so much for the country and created so much bitterness. They were so loyal and never questioned anything until the end of the revolution. It must have been too much to bear and I don't blame him for what he did to me.

'I did not blame him. But he was not the father I wanted. When I beat about 20 million people in the national exam and got a place at Beijing University, he said nothing.

'I associate him with a China we distance ourselves from, that we think of as a nightmare. It is good too for people to read about it and reflect on what one man did and what the whole nation did in that horrible period.'

Life in England, where she has lived since arriving in 1988 to study history at Oxford University, reminded Sun how little she knew about China. She was embarrassed to learn about Xuanzang from an Oxford room-mate from India. As a child, her knowledge of the monk stemmed from the Monkey King children's fable, which depicts him as too weak to travel without the protection of the famous supernatural ape.

'I could have sat in a library and learned about Xuanzang. But I wanted to find out about his belief and why it was being embraced again across China. When I grew up I never knew anything about the history of Buddhism in China. It was attacked so ferociously. There were so few temples. There was really no way to realise the whole story.

'My journey was about bringing back the golden period when China was at the centre of the world. We were so confident of our place that we were welcoming and open to new ideas. And those new ideas contributed to the best period in our history.

'It is a very big story and I could have written a purely historical book. But my filmmaking instincts told me to shape the book into its final form.

'I couldn't leave out my regrets. You can't take back what we did. But I couldn't ignore the way that I really started to feel regret for the way my family had treated my grandmother. She was just an ordinary woman. But she had the faith and she kept it through all that she went through,' Sun says.

'I started to realise the journey could teach me a lot about my grandmother and the experience of so many Chinese people and also about this great man, Xuanzang.'

Sun was unable to start her journey until 1999 because of her work as a producer on the 26-part documentary series on the 20th century, The People's Century.

After spending more than three years on the series, she found a window between projects and convinced her British husband that it was time to leave alone on a trip she expected to take more than a year. 'It was a hugely emotional time and I was reading a lot about the period,' she recalls. 'I knew I just had to go. There were sacrifices. Life is always a compromise. You learn to let things go, and that the things that really matter to you will come back.

'My husband could see what this meant to me. He had seen the direction I was heading through my films. He did not put up too much of a fight.

'The strongest opposition came from my family. My mother said, 'Why are you doing this? Are you not happy with your husband? Have you lost your job?' My brother and sisters thought nobody wanted to know about ancient China. They told me there were so many interesting things happening today in China.

'Chinese people today are very much buried in what is now. Life is full of opportunities for making money but it is still tough. So they don't see the point of the past.

'It was a journey on a broad historical level. It was a journey to discover a history that I did not, and I think most Chinese do not, know. That journey was quite a leap for me, it changed my perception of history and showed me how the true history shaped China and continues to shape China, but also how the perception I had before is also shaping China.'

The other aim was to make sense of family, culture and history. And on a personal level, it meant 'coming to terms with my grandmother's world. It's a world that I can't ever fully enter.'

Sun closed her trip with a rewarding stay in a monastery. But she was unable to commit to Buddhism because she could not accept the idea of karma and rebirth. Her interest in Buddhism, however, grew into reverence during the journey. She started in Xian, the Tang capital, before heading to Xinjiang to visit an aunt who lived in the region for decades as part of the Han Chinese influx.

Sun passed through the Taklamakan desert and saw Kyrgyzstan's Heavenly Mountains. Denied entry to Uzbekistan and Afghanistan, she flew to Peshawar, Pakistan, before seeing the most sacred Buddhist sites of India. The subcontinent taught Sun much about China.

'The year I was born we had the border war with India. This country that was equally ancient and equally wonderful was depicted to me through images of hunger, starvation and natural disasters.

'This civilisation gave us so much and the cross-exchange was so extraordinary. This flow of information, material culture, lasted for a thousand years. It must be one of the greatest exchanges in the history of mankind.'

Sun took the journey with no plan to write a book. But eight years of film work created the habit of reading extensively and keeping detailed diaries. In early 2001, six months after returning to London, she met literary editor Susan Watt at a party.

'I told her I was a filmmaker who had not been working much because I had just been on this crazy journey. I thought it was a bit remote for a British audience, but she said: 'You never know. Write me something.'

'I thought of a Chinese saying: A dragon will give birth to a dragon; a phoenix will give birth to a phoenix. I had the impression that if you are a writer you are born a writer. But she told me to sit down for five days and write what became the first two chapters. It was about 20,000 words.

'She came back three days later and told me to get an agent. My book was my take on that period and my life. In bridging my story and the history of Xuanzang's period, I found that I was a storyteller.'

Sun says she has learned things about Chinese culture and Buddhism that help her now.

'I wanted to show readers that what once made the culture brilliant could give people clues to the future of China. Xuanzang's jump into the centre of the world gave the Chinese a huge leap from their tough daily lives. He appeals because he did things people thought were impossible.'