As the British Home Secretary, Jack Straw, reconsiders an earlier decision to extradite former Chilean dictator Augusto Pinochet to Spain to face charges, Dr Sheila Cassidy finds herself again reliving the torture and solitary confinement she suffered following her arrest by the junta, 25 years ago. "It all happened a long time ago but now with all this fuss, I'm back talking about my time in Chile - funny, isn't it?"

For all the horrors she has lived with and the long legacy of psychological damage, including depression and ongoing insomnia - "I made no connections with what was wrong with me and what had happened earlier until about 1984" - Cassidy, now 61, never lost her sense of humour.

She makes no claims to being an expert on Chile, and remains apolitical - "I'd have to be honest and say politics bore me and I left that Chile thing behind a long time ago" - her experiences left her with an interest in human rights and, although not a natural campaigner, she became a spokesperson. "It's because I'm so articulate - I got asked to speak about Chile and I responded to many of these requests, I still do. I did it for many years. Preach on Human Rights day; address this body; speak to that group. I became a sort of Christian voice, and I am a powerful speaker. But I can tell you, it was extremely exhausting and the endless reciting of my experiences left me seriously depressed." Does she feel Pinochet, now 83, should be extradited? "I think it is very important that he stand trial. It is important for all the people who were tortured and for their families, and especially for the families of the disappeared because they have a great need to see justice done at last."

Calmly - almost neutrally considering she has such a forcefully expressive, animated personality - she adds: "Whatever is done will have a serious impact. It will be an important signal for present-day and retired dictators, letting them know they are not welcome and also that there is no hiding place." Is there an element of revenge in all of this? "It is not a question of revenge, it is about enabling people to get some kind of closure, letting them arrive finally at the end of a chapter of their lives." Medicine, not politics, brought her to South America. "I had friends there and went there really for further surgical experience." In the company of her dog, she travelled on a German cargo boat and arrived in Santiago shortly before Christmas, 1971. "At the time, I spoke no Spanish and began attending at the hospital, more or less like a medical student." She was based in the cardiac unit and remembers being aware of the excitement felt by her colleagues. It was a time of change, and seemed for a while, under Allende, to be one of great hope. Santiago was an exciting city.

"Well for one thing, it was a much warmer city. You had the alfresco life. People sitting, talking, in cafes late at night. Life was exciting from a social point of view. The guys I worked with were really excited about the Allende reforms. Morale was high among the people I knew" - left-wing, middle-class intellectuals and professionals. "Socially, I didn't mix with the poorer people", but many of her colleagues were committed to fighting for the rights of the poor. Idealistic euphoria ended abruptly with the coup on September 11th, 1973. Living in a state of curfew became routine.

No one is more aware than Cassidy of the ironies of being in the wrong place at the wrong time. "Aside from listening to my friends and watching their reactions - I had no interest in what was happening." Nonetheless, she found herself in trouble. "I was asked by a priest I knew, and liked a lot, if I would treat a man with a bullet wound. It was obvious the man could not go to hospital for treatment for fear of arrest." The injured man was Nelson Gutierrez, second in command of the GAP, the left-wing revolutionary forces.

Within a week of treating Gutierrez she was arrested - a house in which she was visiting was raided and a maid was killed. Cassidy remembers looking for something to anoint the dying woman. "There was a jar of Nivea, but I thought that wouldn't be quite suitable; I searched some more and found a bottle of olive oil." Juxtaposing the off-beat and the profound is second nature to Cassidy. Funny and slightly shocking, she comes complete with a store of jokes about religion, her mistakes and the ongoing tension between her spiritual and consumerist selves. Equally, she is conscious of the importance of faith while at the same time acknowledging the failings and the hypocrisies of the Catholic church. She stresses the importance of counselling for children who have been victims of abuse by priests. Addressing the Oscar Romero Human Rights seminar at Trinity College before Easter, she clambers onstage - brisk and very English, almost Mary Poppins-like in demeanour - clutching a large teddy bear, "my moral support". Small but robust, quick-witted and so used to addressing strangers, Cassidy sits on the table and the bear stays on her lap.

Interviewed the following morning she is, as expected, direct and opinionated and one has the impression she would like to appear rather more eccentric than she actually is. On leaving to go to the airport, she takes a wine bottle containing a red rose from the window sill, hands it to me, and bows solemnly: "Here, take this home."

Her humour is black, and at times, quite raw. Too practical to fall into the trap of self-seriousness, she says: "I used to be self righteous but I've given that up now." Describing her arrest, she mentions the terror, what it was like to be shouted at and slapped in the face. "There is this curious outrage at this gratuitous violence." There was also the humiliation of being stripped naked. Three days of torture culminated in having electrodes placed in her vagina. As an older, wiser Cassidy, well used to addressing audiences, she remarks: "If you're interested in the gory bits, read my book."

Audacity to Believe was published in 1977. Since then, she has written six more books and is currently working on her eighth.

Torture was followed by three weeks in solitary confinement and a further five in detention. Cassidy's ordeal resulted in the recall of the British ambassador from Chile.

She herself was expelled from Chile in 1975 and returned to Britain, initially to be an icon of sorts, a witness. "My testimony was important. But I tried to act as if my experiences had not taken over my life - which, in fact, they had."

Nine years of living a hidden life of insomnia, depression, exhaustion and fear followed - "I was always afraid I'd lose my job or that I would be found out for being depressed". It took Cassidy a long time to realise that when you are in charge, people tend to love you "only if you are strong and are anxious when you are weak".

It is easy to imagine her appearing oppressively jolly and unbothered by her imprisonment, while privately she underwent long spells of depression and fretted through sleepless nights. "There is nothing which can unhinge you more than going without sleep. People who don't have insomnia have no notion of what it is like to go without sleep. The night seems so long, but you keep on hoping. You think `maybe if I eat something, maybe if I go to the bathroom again?' Then it starts to get light and there's no hope of sleep. Then it's morning. And people ask: `Well, did you sleep?' Finally I had to do something about it. After years of half living, I realised what was wrong. I had been living in a state created by suffering from post traumatic stress syndrome.

"Anti-depressants restored my sleep, took away the anxiety and depression and made life worth living again." It all seems so far removed from being the last ("let's be completely accurate here - afterthought, mistake") child of three, born in 1937 in Lincolnshire, daughter of Air Vice Marshall John Reginald Cassidy, a central figure in Britain's telecommunications during the second World War. "He was senior lecturer at the Electrical Wireless School in Cranwell. He was a very brilliant man, a brilliant administrator. He was a Catholic, so my mother had to convert to marry him." Was she close to her father? "You could say I had a love/hate relationship with my father." It seems as if there were two families. "My sister is 13 years older than me, my brother is six." Her childhood was middle class - "we liked to think of us as upper middle class" - and it was also rootless as her father's career took the family at various stages through a long lists of air bases around Britain.

When Cassidy was 10, the family moved to a chicken-farm in Australia. "Initially, the plan had been to go sheep farming, but that didn't work out. When we were almost on our way home we discovered a chicken farm, with 2,000 chickens. For a while I had a horse. It had strayed and we didn't try very hard to find its owners. I had it for about six weeks. I had a very happy childhood."

By the age of 15, she realised she most wanted to be a doctor. Having begun her medical studies at Sydney University, she transferred to Somerville College, Oxford when her parents returned to England in 1958. "I love the drama of medicine," she says. "I particularly like accident work, I love hospitals. I used to love the smell of ether."

Within a fortnight of her release in 1975, Cassidy was on her way to Geneva at the request of the UN Association to give an address on her experiences. It was the beginning of an intensive period of talking, designed to lift the general awareness of the situation in Chile and in the process to highlight crimes against humanity occurring elsewhere.

Cassidy had long struggled with issues of faith. During her time in solitary confinement in 1975 she "celebrated a do-it-yourself Eucharist with bread and water and made an awesome covenant with God to do what He or She wanted for the rest of my life". In 1978, she spent 18 months on the periphery of Ampleforth Abbey, attending the Divine Office with the monks and having lessons in scripture." This went so well, it inspired her to enter a monastic house for women and begin an "apprenticeship" there. It proved a disaster. "I was thrown out." Not because she was disruptive, but because "I was so miserable, so unsuited to the life. I'm sociable, but not very good in cloistered environment. I'm afraid I'm given to nit- picking."

Even so, her reaction to being asked to leave was anger rather than despair. "I was very cross, miffed, and promptly set myself up as a hermit." It seemed like a good idea. "But I ran out of money in six weeks and realised I had to stop playing nuns and go back to the one thing I could do for money - doctoring. And frankly, I love being a doctor."

On completing a month-long locum in the oncology department at the then Plymouth, now Derriford General Hospital, "I was invited to stay - and stayed for 18 months".

The next step was as unexpected as it was exciting, she was offered the job of medical director at a new hospice "near the hospital but independent of it". She worked there for 11 years, continuing a part-time involvement with the general hospital during which she held clinics and did ward visits, mainly with cancer patients.

In 1992, however, this ended. "I was told to leave." No reason was given. "They were making me redundant because I was away lecturing too much." Cassidy simply returned full-time to the hospital. "I just took my work with the dying back to the main hospital." Four years later, a palliative-care consultant - not her - was appointed. "I never did any post-graduate courses, so I couldn't be a consultant. Anyhow, by that time I was particularly involved in the psychological needs of all cancer patients, not just the dying."

She began Jeremiah's Journey, a group programme for helping bereaved children, in 1996. "We perceived the need in Plymouth." It is similar to other programmes developing throughout the UK and was partly inspired by a story Cassidy wrote about a family of bears in which the mother dies. It was a teaching aid for a course she was giving to nurses: the name came from one of her bears - she has a collection of 200 teddies and points to a recent acquisition, which is small, and would be quite anonymous but for its beautiful costume - "I bought it at an airport. It's very fashionable in England to name things after teddies," she says.

Three years before founding the children's group, she had begun the Mustard Tree, a cancer-support drop-in centre, "We see about 800 patients a month. We have a large, purpose-built building. There are a number of therapies - massage, reflexology and various craft groups."

As a child, she was encouraged to work with her hands. "I come from a family of artists - all the women in my family are very interested in their houses. I could have been interior designer: my flat is wonderful. I enjoy it, I like having busy hands. I've always altered my clothes . . . clothes are very important to me." Among her hobbies is watching television, "at least three hours a day", particularly police and hospital dramas. And there are the teddies.

Cassidy lives alone: her daily work at the hospital includes seeing individual patients, running groups, visiting patients in Fighting Spirit, the breast cancer clinic she founded, and working on a team that teaches communication skills to doctors and nurses who must break bad news to patients and their families.

She looks well, younger than her years: "Thank you. It's all due to anti-depressants and HRT." While she enjoys telling listeners "I was thrown out of Chile, thrown out of the convent and sacked from the hospice", she agrees she has an impressive record in starting very useful organisations. "I'm a great starter, a great visionary," said with mock but deliberate humour, "but I'm a poor maintainer. I'm a dreamer. I get another idea, and my energies fly off in a new direction."