Small coincidences have big consequences. It's a truism, but in writing my book Philomena – now made into a startlingly moving film by Stephen Frears, Steve Coogan and Judi Dench – two events showed how true a truism can be.
The first concerned myself and an unexpected meeting at a party in 2004. I'd recently lost my job as a UK government communications director after a row with Alastair Campbell and Stephen Byers and I was at a loose end.
A woman I didn't know said she had a message for me from a friend. She knew I'd worked for many years as a BBC foreign correspondent and she asked if I'd help her solve a family mystery. It set me on a five-year search for a man I'd never met.
I met the friend, Jane, for a coffee at the British Library.
She said her mother, Philomena, had revealed a secret she had kept for 50 years – she had a son she had never spoken about to anyone. Philomena had been a teenage single mother in Ireland at a time when sex outside marriage was considered a sin. Jane said her lost brother would now be in his early fifties and they were both desperate to find him.
I was intrigued, but my journalism had been about wars, international crises and superpower summits. I wasn't sure about writing a human-interest story. What convinced me to do so was meeting Philomena herself. She was friendly, bright and likeable – qualities in short supply in Westminster and Whitehall. We hit it off straight away.
Philomena told me she had given birth in a country convent at Roscrea in County Tipperary on July 5, 1952. She was just 18 when she'd met a young man who bought her a toffee apple on a warm autumn evening at the county fair.
"I had just left convent school," she remembered. "I went in there when my mother died, when I was six and a half, and I left at 18 not knowing a thing about the facts of life. I didn't know where babies came from."
Philomena's family had "put her away" with the nuns. And after her baby, Anthony, was born, the Mother Superior warned she would face damnation if ever she revealed her guilty secret.
"All my life I couldn't tell anyone," Philomena told me. "We were so browbeaten, it was such a sin. It was an awful thing to have a baby out of wedlock."
Philomena was one of thousands of Irish women sent to convents in the '50s and '60s, labelled moral degenerates by a Church hierarchy who ruled that they must not be allowed to keep their children. For the Church, it was a lucrative deal. The Government paid a pound a week for every woman in the Church's care, and two shillings and sixpence for every baby.
After giving birth, the girls were allowed to leave only if their family could pay £100. Those who couldn't afford it – the majority – were kept in the convent for three years, working in kitchens and laundries, or making rosary beads, while the Church kept the profits from their labour.
Philomena served her three-year sentence in the laundry at Roscrea and caring for her son.
But at Christmas 1955, she was told Anthony would be given up for adoption. "Oh, Anthony was gorgeous," Philomena told me. "He was a lovely, gentle, quiet lad. All my life I never forgot him. I prayed for him every day. I just want so badly to know what happened to him."
Discovering what happened to Anthony Lee was the result of the second chance encounter in this tale. Early in my search, I realised that the Irish Catholic hierarchy had been engaged in what amounted to an illicit baby trade.
The "orphans" were sold to the highest bidder, many bought by wealthy Americans for $2,000 to $3,000. It took patient detective work – not helped by an obstructive Church – to discover that Philomena's son was one of them.
But by piecing together passport records, archived documents and fleeting references in old newspapers, we found that Anthony Lee had been offered to an American Catholic couple, Marge and Doc Hess, from St Louis, Missouri. In my research, I came into possession of Marge's diary from 1955, which revealed that the Hesses had three sons and wanted to adopt a daughter.
Marge Hess had flown to Ireland looking for a little girl. Her diary tells how she found three-year-old Mary McDonald in the convent at Roscrea; and it recounts the coincidence that led her to adopt Anthony Lee.
When Marge leant down to pick up her new daughter in the nursery, a little boy came running to give her a kiss. He was Mary's best friend, and Marge fell for him at once.
She rang her husband to ask if it would be okay to bring two children back to America. Anthony's show of affection was the nudge of fate that transformed his life. He and Mary were whisked from rural Ireland to the New World.
Renamed Michael Hess, he grew up to be a handsome, intelligent young man with a successful career as a lawyer. Ronald Reagan's Republican Party brought him into the White House and when George Bush Senior became president, he made Michael his chief legal counsel.
But Michael Hess was a gay man. Obliged to conceal his sexuality in a homophobic Republican Party, he was tormented by his double life. He was tortured, too, by the absence of his mother and by the orphan's sense of helplessness: he didn't know where he came from, who he was or how he should live. He went back to Roscrea to plead with the nuns to tell him how to find his mother, but they turned him away.
In the late 1980s, Michael Hess became infected with HIV. He kept his illness secret, but in 1993 he went again to Roscrea to appeal for help. The nuns again refused to tell him where he could find his mother, or indeed that her sisters and brother – his aunts and uncle – were living just a few miles down the road.
In desperation and with deteriorating health, Michael asked the Mother Superior if he could be buried in the convent if he were to die: he would put enough information on his gravestone to help his mother find out "if ever she comes looking for me".
As we discovered, the nuns did agree to let Michael be buried in Roscrea – for a large donation to Church funds – and he did put a "message from beyond the grave" on his marble headstone, a message that ultimately allowed us to trace the path of his life.
"Michael Hess, a man of two nations and many talents," the inscription reads. "Born July 5, 1952, Sean Ross Abbey, Roscrea. Died August 15, 1995, Washington DC."
The film depicts our journey in search of Philomena's lost son. It reveals the tragedy of a mother and child who spent decades yearning to find each other, thwarted by the nuns' refusal to give information.
There are thousands of other lost "orphans" whose lives were similarly blighted. Today many are still looking for their parents and, through them, for their identities.
Philomena is a remarkable woman, extraordinarily devoid of bitterness.
After her son was taken from her, she moved to England, where she trained as a nurse, married and had two children. We will go together tomorrow to see the film premiere at Leicester Square in London.
Despite the decades of heartache, she blames herself for everything – for giving her son away and for not speaking out earlier.
"If only I'd mentioned it all those years ago. Oh Lord, it makes my heart ache," she says. "I'm sure there are lots of women to this very day – they're the same as me; they haven't said anything. It is the biggest regret of my life."
* Philomena by Martin Sixsmith (Pan Books) l The film will go on general release on November 1