"My colleagues and I laugh … all that cleavage, perfect hair and Hollywood teeth, great-looking blokes, high heels. Have they seen the sterile paper suits we wear? I remember it was so cold in the London mortuary once that we had five or six of them on. Do they know what people really look like with those things on?"

Unusually, she is at home - and praying that the telephone will stay silent long enough for her to enjoy at least part of the British summer break.

Death, indeed, mass graves are MacKinnon's passion but also her bread and butter. Conjure a recent human disaster and chances are that this forthright, quietly spoken blonde was there.

Bosnia and Herzegovina and Croatia? Yep, she was there for three years in the field, between spring and when the snows came, collecting evidence of mass atrocities with the forensic teams of the International Criminal Tribunal for the former Yugoslavia.

MacKinnon spent more than six months in New York after the September 11, 2001 attacks, first as a locum when an anthropologist friend was injured in the tower collapse then with a victim identification mortuary team entrusted with the overwhelming job of re-opening more than 18,000 body bags to try to identify, DNA-test and, finally, separate the tragic mess of co-mingled human remains.

Suddenly, mid-sentence, she flicks the laptop to reveal a new screen and folder of photographs. The enormity of the job is beyond comment: "The bags were often not body bags but buckets … see that line? They were the containers, hundreds and hundreds of them inside the big refrigerated containers parked in rows. I had lived there, had friends there …"

Earlier this year, a call summoned her to the Isle of Jersey to peer into the darkness of the still-to-be resolved Haute de la Garenne child-abuse investigation. And just last month, MacKinnon and the team led by Dr Tony Pollard, from Glasgow University, finalised a still-to-be released report to the Australian Government which wrapped up the exploratory excavation of the five mass graves at Fromelles, north of the Somme Valley. Their team was instrumental in verifying the existence of 300 or so World War I diggers, both Australian and British, who have lain, lost for 90 years in German-dug pits in a field next to the picturesque Pheasant Wood.

Why?

"I see my work as a way of giving a voice to the dead." she says simply.

Fit, an outdoor lover and avid trekker, MacKinnon, 47, is one of four children of a Victorian farming family. Their ancestral origins lie on the Isle of Skye, hence her emotional attachment to Scotland. As a young girl, she remembers a passion for medicine but home was full of kids and chaos. She sought refuge with the next-door neighbours and was allowed to spend hours immersed in medical and anatomy texts. "Looking back, I think I have always had a fascination with the dead and post-death processes," she says.

Of her 18 cousins, MacKinnon ended up being the first to attend university but did so late. She spent time travelling before embarking on what was to become a 10-year stint in the film industry beginning with a job as a typist at a Melbourne production company - "I was the blonde on the desk" - and on to production and location work with casting director, Rhonda Schepisi, who became a friend and mentor. A stint in Perth was followed by a move to London - "Yes, I went for a man" - and she ended up marrying the bloke's best mate. "No, it didn't go down so well," she laughs.

In her early 30s, MacKinnon conceded that film wasn't her calling and enrolled in archaeology at University College London. Her studies began in Western Asiatic archaeology but it did not take long for her to uncover the old passion for human remains analysis.

When both her grandfather and Schepisi died within a month of each other, however, an existential crisis flared: her marriage ended just weeks before her final exams and while she passed splendidly, MacKinnon moved to embark on a new life and a master's degree in Sheffield. This time it was osteology, paleopathology and funerary archaeology. An 80,000-word thesis (the limit was 20,000) which analysed weapon trauma on skeletons buried in a medieval cemetery in York crystallised her professional trajectory and she began to think about human rights work. "I got to a point where I simply couldn't deal with living people … I hate seeing people in pain you know … I just can't deal with it."

MacKinnon was digging in the Shetland Islands when the first, life-changing call came through in 1998 asking her to join an international mortuary and in-the-field archaeological team seconded to assemble forensic evidence of the atrocities committed during the Balkan wars. (Over several years, they documented evidence for the criminal trials at the Hague and managed to return thousands of bodies for burial to the devastated families left behind.)

"I'd been banging on about it and now I had to put my money where my mouth was … I went straight away but remember on the first day, I stood at the door of the mortuary and honestly wondered if I could walk through and do this."

She did.

And she has not hesitated since.

MacKinnon's fascination with death is both palpable and intriguing. Her stories of witnessing the aftermath of human atrocities, modern and ancient, are told with compassion and, occasionally, the black humour born of tension. Photographs of ancient, but brilliantly preserved remains are showed and shared with genuine alacrity while stories of the horrors inflicted on innocents in places such as the former Yugoslavia, Iraq and the Congo elicit unadulterated but well-modulated emotion.

She speaks with enormous respect for the Argentine and Guatemalan anthropologists and archaeologists who pioneered this kind of forensic work in the 1980s, often risking their own lives in a bid to find, identify and return the victims of criminal, despotic regimes.

MacKinnon also cannot dampen her glee when she taps into her laptop to find an example of the contents of a particularly well-preserved Egyptian grave from Greco-Roman times (between 300BC and 300AD). "Look, see there is the grave cut … there, that is the woven mat that still lies over her, the tiny bones, her sandals … the level of preservation is extraordinary.

"See here, she must have been heavily pregnant when she died. During decomposition the baby was expelled from her body … this other one is pre-Dynastic, the period before the Egyptian kingdoms, five-and-a-half thousands years old … look at her teeth, perfect, that is hair, dessicated skin …"

She pauses: "She really affected me, she was the last one [found] on the last day. It always happens like that … her skull shows blunt-force trauma. It was caved in."

A gentle silence and back to Bosnia, her most formative experience. "I don't want to sound trite but it showed me what I feel I was meant to do. Seeing men executed in situ in a grave is very different to working in the mortuary, in a semi-sterile environment where you open body bags and there are the remains."

The most unforgettable moment was work done in an old quarry. "It was bizarre, they had executed the men and simply back-filled with broken glass from bottle factory. It was not a grave just a big pit covered over … a sea of glass, the bodies, the river flowing beside it."

Often, the smallest things are indelibly imprinted in memory: the Digger in Belgium found with his boots on; the little boys in Bosnia who had been shot on the way home from school - the archaeologists found they still had their homework sheets tucked into their pants.

"The one good thing about Bosnia is we were never told the circumstances of what we were investigating. It meant we could be completely objective, we knew the barest data … archaeologically you go in with no preconceptions. We don't hear the stories, the survivor testimony. I cannot hear that and do this work. I just couldn't."

"I remember one night, an investigator was talking at dinner … these people had been shot and this wee boy had survived under the stack … he ran to the nearest farmhouse for help … but it was where the perpetrators were."

MacKinnon's voice falters almost imperceptibly: "They tortured him, took him back to the site, killed him there … you cannot hear that stuff. You just cannot. The only distance you can operate with is, 'Don't tell me, let me do my job, let me see what I come up with without circumstances of the death and deposition'."

Thoughtful, engaged, MacKinnon observes that working on disasters such as the World Trade Centre may provide different political or human contexts but what drives her to do her job is the same. Human beings need to reconnect and somehow bid farewell to those who have been taken away, by accident, by violence and politics or sheer malevolence. "I can't say it is less stressful, just different. Obviously, it has a different poignancy when you are looking at executed children … or old men or young men."

It comes naturally to ask if it is ever possible to leave such experiences behind. With a growing memory bank of such tragedy, how does she live day to day? MacKinnon doesn't hesitate, not even for a second: "You cannot leave it behind because that would not be bearing witness then would it?

"But obviously, you find ways of processing that information and putting it in a place where it stays alive."

She stands up and we take a walk through her home, graceful, meticulously renovated, once the old laundry of the earl's castle up the way. A historic bridge, complete with half-moon ridges where Jacobites stopped to sharpen their swords is framed in her kitchen window. A river flows at the bottom of the garden. It says much about her life choice and the need for peace - outside work.

Afterwards, back at the table, MacKinnon returns to the coping issue. "I don't go back there … unless of course I am asked or start talking as I am with you. There are triggers too … like smells … I have had quite a lot of trouble with some foods, I cannot eat meat with bones or tubes, I can't, anymore, eat the kind of ripe French cheeses like brie that gives off a smell of ammonia … it is a smell associated with decomposition," she says with resignation.

That night, we have dinner and the Spanair flight comes down in Madrid burning 153 holidaymakers to death. She is matter of fact: "The Spanish are very good at this stuff since the Madrid terrorist train attack, they don't need us."

After, we drive through the disquieting blackness, under centenarian trees, to her home. Unperturbed, she waves gaily and disappears down the ancient stone steps into the gloom.

It is only later, alone, that I remember a surprising, off the cuff remark she made earlier

"You know, my house is haunted …"