Tonight, on the 25th anniversary of Pan Am Flight 103 falling to earth over Lockerbie, Dr Jim Swire will, at last, walk away from his lengthy search for the truth.

Mirroring the precise time of the worst mass murder in British history, which took place on December 21 1988, he will attend a service in Westminster Abbey to commemorate the 270 dead, whose number included his beloved older daughter, 23-year-old Flora MacDonald Margaret Swire. She died on the eve of her 24th birthday.

A medical student first and foremost, Flora was musical, singing in operas, playing the guitar and piano. She sculpted. She painted. She'd started writing a novel. She was an extrovert, fun to be around and, as the big sister, pivotal in terms of family dynamics. Jim feels keenly that Flora's death was a great loss to medicine. When she died, she had already become distinguished for someone so young. Flora had been invited to work at the UK's top neurology institute in London's Queen's Square. It was a dream come true. She had completed her pre-clinical training at Nottingham University, intent on becoming a neurologist.

Her father is a man for whom the word conspiracy might have been invented. He will tell you repeatedly that Flora bought her plane ticket just 24 hours before departure after she had been told there were no seats to New York before Christmas. Suddenly there were. It was a last-minute decision to spend Christmas with her boyfriend, Hart Lidov, in America.

It is thought that, in his three-minute speech this evening, Jim, who has been at the forefront of the campaign for justice for the Lockerbie dead and their families, will hand the baton to another prominent supporter of the victims' families, whose role will be to prove that the evidence against the convicted Libyan bomber, Abdelbaset Ali Mohmed al-Megrahi, does not hold water.

These 25 years have taken their toll on gentle Jim. He is an infinitely approachable man, full of integrity, with huge, saucer-like blue eyes and a rugged, handsome face, beneath a shock of white hair. Educated at Eton, he enjoyed the life of a successful country doctor, in Worcestershire, in a house he cherished, surrounded by church spires and leafy lanes. Back then, Jim believed governments did what they said they were doing. Now he knows better. These days, the man is about as anti-establishment as a doctor could be, one reason why his campaigning lost him his job and pension.

Much to the delight of his wife, Jane, a former RE teacher, Jim has decided, at 76, to take a step back to enjoy what remains of his extraordinary life. In 2003, the Swires downsized from their country home with its 17 acres and bought a smaller house in the Cotswolds town of Chipping Camden. They return to their old home to visit Flora's Wood, on the A38 to Bromsgrove, where, shortly after the bombing, Jim planted, 1500 young trees, oak, ash, chestnut and beech, shaped in a large F, a permanent memorial to his murdered daughter. The Swires are now blessed with four grandchildren. Their second daughter, Catherine, 46, is mother to Grace, 11, and Sam, 9, and their son, William, 41, is father to Oscar, 11, and Lewis, 7.

On the night of December 21, 1988, the couple heard a plane was down and spent that entire night desperately trawling for information. They dialled and re-dialled a help desk in New York to ask whether Flora had been on the plane. A woman said "I don't know" and put down the phone. It was to be Jim's first, unforgettable, contact, with the impenetrable wall that is official indifference. The following morning they discovered there had been no survivors; that, on the day of Flora's 24th birthday, what remained of their first-born lay scattered over some remote Scottish hillside. Instead of celebrating with a cake and presents, the Swires had to cope when TV cameras and journalists from all over the world appeared like an unsightly rash around their sandstone farmhouse.

Even before Lockerbie, Scotland had always played an important part in their lives - Jim grew up on Skye. Named after her father's direct ancestor, Flora Macdonald, his daughter's ashes were scattered in Caroy graveyard there, overlooking the water in which she had paddled as a little girl. No doubt, her family will reflect that Flora would have been 48 now, a neurologist, a mother and wife. Who knows what else?

Many bereaved parents would have succumbed to their grief long before now. At first, Jane tortured herself reading everything she could about what happens when a plane explodes at altitude. She wanted to know. When she learned that passengers might have experienced 15 seconds of consciousness, she sat alone in her kitchen, timing the period again and again on the kitchen clock; watching the black timer tick away the seconds Flora had to endure. She prayed her daughter felt neither terror nor pain.

"I think it must have been mercifully quick," she told me, as if trying to persuade herself. "But I've had a battle with guilt. The fact that I wasn't there to hold Flora's hand during those final terrible moments was like being buried alive. You are there at the birth of your child. You need to be with them at the end, if it comes, unnaturally, before your own."

Since that agonising day, there is nothing about the horrors of December 21, 1988, her husband refuses to confront. An expert on weapons, bombs and terrorism, Jim is familiar with every security error, every attempt at buck-passing. His quarter century campaign has provided an outlet, not just for his grief but for his burning rage. "You can call me obsessed if you want to," he says. "A lot of people do. But obsession is such a negative term. It suggests someone who is irrational. No one ever bothers to explain the difference between determination and obsession."

In Lockerbie, the Swires were told that, on the advice of a psychiatrist, relatives couldn't see the deceased. Desperate to bid Flora farewell, the fact that Jim was a doctor persuaded the consultant in charge of post mortems. Part of Flora's head was missing, her face horribly disfigured after being thrown from the plane at 31,000 feet. It was a traumatic event, even for a doctor.

While Jim sought relief in campaigning, Jane has been driven by a determination to hold her remaining family together. She recognises that her remarkable husband has, against all odds, refused to give up. Despite his grief, he has confronted, with incalculable courage, some of the world's most intimidating leaders, including Colonel Gaddafi, with whom he became friends, the point of unity being that the two men had both lost precious daughters.

The pain is forever etched across his face. Opposite the Swires's bed hangs a portrait painted by the father of Flora's American boyfriend. In her right hand, the 23-year-old carries wild flowers, in her left a sprig of forget-me-nots. It is the first thing Jim and Jane see in the morning and the last thing they see at night. Their grief will never go away.