Our daughter was killed. Then they destroyed her.

January 2000. The new millennium; the new start. But within days, things were going badly wrong for Mary and John Barclay. It was at 7.20pm on January 4 that the telephone in their Peterhead home had rung. It was their son-in-law, Niall McDonald. Mandy was missing. She had left after a row at 2am and hadn’t been seen since. Was she with them? No, they hadn’t seen her. What was the row about? "Oh, you know Mandy," McDonald said vaguely.

Mary advised him to phone the police immediately. But at that point she assumed her daughter was probably at a friend’s. She didn’t know that in Mandy’s Westhill home, all the things she would have taken had she left the house were still lying around: her purse; her asthma inhaler; her eczema cream; the specialist contact lenses that she could see little without. "What was she wearing?" Mary asked. "Her Aztec patterned coat," said McDonald.

Mandy didn’t return the next day. Mary and her sister moved into her home to help Niall with the children. On the first night he handed Mary a hospital appointment card for Jenny. "Here," he said, "you’d better take this." It wasn’t for another two weeks. Mary quietly put it back behind the clock.

Niall never joined the search parties that went out for the next few nights. "I thought, ‘Poor thing,’" says Mary. "‘He’s waiting for a phone call.’" As they shone their torches into the darkness, trying to shed light on what had happened to 32-year-old Mandy, Mary’s husband John kept veering off, talking to the police. "He came back," recalls Mary, "and said, ‘You know, the police say they look at the husband first.’ I said, ‘You can’t go around saying things like that!’" John protested that he was only repeating what he had heard. "Wheesht, now," Mary insisted. "You canna say that…"

It was six days after Mandy disappeared that the Barclays’ doorbell rang at half past midnight. It wasn’t good news. The police had found Mandy’s body and arrested her 35-year-old husband. "And what did I say?" recalls Mary. "I said, ‘Why have you done that?’ I don’t remember if I said, or just thought, that they had simply latched on to the nearest one they could find."

But detective chief inspector Sandy Kelman, who led the inquiry, explained why the arrest had been made. It was Niall McDonald himself who had taken them to the woodland, right to the spot where Mandy’s body lay dumped like discarded rubbish, her Aztec patterned coat lying beside her.

It’s hard to comfort someone down a telephone line. Not hard to know they’re hurting, though, because the voice acts as a conduit for the emotions. Four years after Mandy’s murder, the day after Niall McDonald has finally lost his appeal and been jailed for seven years, I reach Mary Barclay on her mobile phone. She’s on her own, out driving in the country roads. Going nowhere in particular, she says. Just driving and clearing her head. She’s delighted by the previous day’s verdict. McDonald spent only six months in jail before being bailed on appeal. Now he is behind bars, where he deserves to be. But Mary’s voice isn’t registering delight. It sounds raw and sore, and finally breaks. "I just feel," she says through tears, "that my daughter’s reputation has been ruined."

Mandy’s body, the shell of her, was destroyed by brute force. But it was the lawyers in the courtroom, wielding words like weapons, who destroyed the rest of her, the person that she was. Niall McDonald never took the stand. So who was this woman who emerged from the court case, a woman who indulged in kinky sex, who wore French maid outfits and used whips as sex aids? She wasn’t recognised by any of Mandy’s family. Was this person Mandy’s alter ego, a woman they never knew? Or was she, perhaps, just a make-believe person, a woman created as a defence for the man who admitted killing her?

After Mandy’s body was found, Niall McDonald never denied responsibility for her death. But he claimed it was an accident, a sex game gone wrong. On the night of January 3, 2000, the couple’s two children were staying with friends. He and Mandy had been partying and, according to McDonald, came home and had sex. McDonald claimed he climaxed but continued to stimulate Mandy, putting his arm over her neck to heighten sexual pleasure and using a whip to penetrate her anally.

This was sex that "narrow-minded people would call kinky", said McDonald’s lawyer, Donald Findlay QC. Kinky? Court papers show Mandy had injuries on her face, neck, vulva and anus. She died of asphyxia and rectal injuries so severe her bowel had been punctured. "The damage to the bowel was consistent with blunt instrument trauma penetrating through the anus and delivered to the bowel wall forcefully enough to occasion haemorrhage."

The jury convicted McDonald of culpable homicide, rather than murder. Mandy’s body had lain outside for six days, and experts said vital forensic evidence had been destroyed. But the conviction made clear that the jury did not fully believe McDonald’s explanations. This month, appeal judges upheld the decision, saying, "In our opinion, there was evidence which entitled the jury to draw the inference that the appellant did have the intention to cause physical injury, and that his intent was not limited to the infliction of pain."

There were always discrepancies to McDonald’s story. He claimed at one point that Mandy had complained sex was painful. Why, then, would she agree to penetration by a rigid whip to her severe injury? McDonald’s arm over her neck was supposedly some form of erotic asphyxia. But studies suggest erotic asphyxia is predominantly, though not exclusively, a male predilection and usually involves some form of ligature. And why was Mandy’s body dumped for six days? "There are a number of issues his explanations did not adequately clarify," says DCI Kelman. "If someone has an accident in the circumstances he speaks about, the first thing a loving spouse would do is get medical help. His explanations did not fit."

Ultimately, it does not matter if Mandy McDonald was sexually adventurous or not. She didn’t deserve to die. But the deliberate creation of the suburban housewife dressed in a saucy outfit behind the net curtains of her middle-class home was important. It diluted sympathy for Mandy, deliberately turning her from a victim into a woman who regularly played with fire and had finally got burned. It was a hugely successful manipulation. It is Niall McDonald’s picture of Mandy that remains hanging on the walls of public consciousness, despite its artist having been discredited.

There are only two people who can lift the lid completely on events at 1 Arnhill Drive in Westhill, and one of them is dead. But Mary and John Barclay, and Mandy’s two daughters, Catherine and Jenny, believe the true story was not presented in court. Now it’s their turn to paint the picture.

Mandy and Niall McDonald, defence lawyers insisted, were a loving couple. But behind closed doors, their marriage was in serious difficulty and the couple were weighed down by severe financial problems. Temperamentally, they were different. Mandy was only 4ft10in but she was sparky and lively with a fiery temper. Niall was quiet and dour.

Mandy left school at 16 and married young. She had been in a succession of jobs but was always ambitious to improve her life. She encouraged Niall to study with the Open University while working at a local paper mill, but he soon dropped out. Mandy decided it was her turn. She studied for qualifications that enabled her to enroll at Aberdeen University to study engineering, like her father. She completed two years and got good grades, but her parents noticed that the better she did the worse things got at home.

Mandy worked part-time in a supermarket and had put money aside for her third year, when she would have to study more. Niall agreed to help in the house, but never did. The only time Mary saw him washing was when she arrived at the house after Mandy died. He was washing sheets. He didn’t sleep the three nights she was there. Later, Mary would be glad Jenny had wanted her to sleep in her room. Niall had offered her his and Mandy’s bed.

When Mandy’s studying took off, Niall suddenly gave up his job. He used his wife’s study fund to retrain as a driving instructor, but consistently failed the final exam. Then Mandy’s brother paid around 1,000 for him to do an off-shore course. But despite being described in press reports as an "off-shore worker", he had never actually taken a job. "I think he was jealous," says Mary. "It was when Mandy looked as though she would go the full course that he started looking for ways to cut the money, trying to force her out financially."

For five months, John and Mary paid Mandy’s mortgage, using John’s retirement money. As Christmas 1999 approached, Mary knew things couldn’t continue. "I said, ‘I think we should come over and clear everything you owe and give you a new start’. My son said he would help and put in 1,500. We went over and made a separate cheque out on the table for each thing. With everything we paid over the months, it must have come to about 8,000. So we said, ‘Mandy, your dad’s retirement money is running down. We won’t be able to do this much longer.’"

They thought Niall was out, but when the cheque signing was over he came into the room and sat down morosely. "He wasn’t a happy man. He never even said thank you," says Mary. Mandy told them she had secured a grant. If her parents could just pay the December mortgage, by January she could manage. "I think with those words, she sealed her fate," says Mary.

The Barclays are thoughtful when asked their opinion of their son-in-law in the 13 years he was in their family. "I didn’t dislike him," says John. "I thought about it later," says Mary, "and I thought, ‘What kind of relationship did we have with him?’ And I realised that because Mandy was bubbly, she did all the speaking. He would come in and you’d ask how he was and he’d shrug and say, ‘Aye well, you know…’ He would sit with his head in a paper and ignore everything, really."

Once, when they were all going out, Mary arrived at their house without McDonald realising and she heard him F-ing and C-ing at Mandy. He stopped abruptly when Mary appeared. Later, Jenny would say it was a common pattern. "She said he would be horrible to them when no one was in, shouting at them, and their mummy would be crying and they would be crying and they’d be sent to their beds crying for their mum. Jenny would shut herself in the bedroom with the lights out. He shouted and roared when it was just the three women but was nice to anyone who came to the house. Mandy covered that all up. She had every right to, as a wife. But we were pretty sick to think they had all suffered this."

Mandy was loyal. Only once did she hint at problems. Mary gets tearful remembering, because every time she thinks of the conversation she wishes she could rewrite it. "She said to me one summer’s day, ‘I told the children that Daddy might not always be with us, it might just be Mummy and the girls.’ And that was all that was said. That was a big thing for her to say. But because she was so private, you didn’t dare go any further. If only I’d said to her, ‘Look, if it’s too much, just pack up and come home.’"

Mary believes Niall McDonald came from a home where he was not encouraged to have opinions. Looking back, maybe his lively, intelligent wife made him feel inadequate. Certainly, within a few months of Mandy’s death he had formed a relationship with a girl who, at 17, was young enough to be his daughter. "I am angry with myself that I didn’t see what he was," says Mary. But is it possible that she didn’t see Mandy as she really was either? The Barclays have a strong Christian faith. Perhaps Mandy kept part of herself hidden. "I’ve thought about it," says Mary. "But I don’t think so. There was nothing in her lifestyle to suggest it." Take the French maid’s outfit. Hardly a sign of debauchery anyway, but the truth is Mandy had worked as a waitress. She wore a white apron and black lace top with silk underneath.

She had also suffered severe eczema since childhood. The point about that, says Mary, is that she was physically shy. She could never be persuaded to wear a bikini or even go in the pool when she was on holiday. "You wouldn’t have got Mandy to uncover herself," she says. "The cortisone had caused scarring; she had a tanned complexion and you could see these white bits. Someone who is an exhibitionist, or wanting attention… you never saw that in Mandy. When she was young she liked motorbikes, but she ended up a very middle-of-the-road housewife who wouldn’t let her children do this and that."

Then there was the whip, the supposed "sex aid". A sex aid, particularly in a house with children, would be kept private. The whip lay openly in the McDonalds’ house and actually belonged to McDonald’s brother. Mary’s son had seen it. Her husband. Her daughter. Her son-in-law. McDonald was once seen tormenting the children with it, and an exasperated Mandy told him to get rid of it before someone got hurt.

There is no evidence, says Mary, that Mandy knew of any of the other items found in the house. "The police found a wife-swapping magazine in the bin," she says. "Was it something he had got rid of after he killed her? Or was it something he got and they had a laugh about? We don’t know. As for the pornography on the computer, we don’t know that she knew what was there. I’m not going to say she didn’t, because I just don’t know."

Mandy was the Barclays’ daughter. They loved her. They would love her whether she had a French maid’s outfit or not, or whether, as was claimed, she had sex on a garage roof or not. Now they go on loving her through her two daughters, Catherine and Jenny, who live with them. John was retired but went back to work full-time to support them; their own father has contributed nothing to their keep.

It has been tough. Mary, like Mandy was, is tiny. She seems strong and determined and vulnerable all at the same time. Her doctor wanted her to go and see a counsellor once. She refused. "Not because I have nothing to say. Not because I am not grieving. But I have my friends; I have my church; I have myself. But I’ll tell you something beyond that. I can’t afford to go opening up and pouring out everything that is in me. There would be no putting me back together."

Catherine and Jenny McDonald sit in the spick and span living room of the Peterhead house that is now their home. Catherine is 12 and looks like her mum. She is lively and engaging but, naturally, she has had her outbursts of crying. She wants to be a lawyer, but only a prosecuting lawyer. She couldn’t bear defending a guilty person. But what if she put an innocent person in jail? "They’re not often innocent," she says.

Jenny is 16, small and slender with porcelain skin, like a little china doll. Her smile is shy and she has a gentleness that makes her seem very fragile. She looks younger than her years, but has a maturity of experience way beyond others her age. She is studying psychology at school and wants to help teenagers with problems. Jenny knows about problems. She has been in hospital with anorexia and bulimia. She has also taken an overdose. "I just reached the point where I couldn’t cope," she says. "It was too hard."

Mary prays for Niall McDonald’s salvation. "I will know if he is saved because he will be the first to put the record straight," she says. But what about Jenny? Could she love her father again? "I thought I could for a little while. All this thing about forgiving everyone. But then you read what happened and you just think, ‘No way.’" Her dad was "a bit rough", she says. Shouting? She nods. "But you couldn’t have imagined him doing anything so terrible." She hasn’t seen him for two years, shakes her head at the thought of seeing him again. "I’ve tried before and it makes me ill. Last time I saw him I lost 10 pounds in five weeks. It’s too stressful."

When Mary lays flowers where Mandy was found, it is in remembrance of the fact, not that she is gone, but that she is with them still. "We haven’t lost Mandy. We’ve lost her company and the children have lost their mum to bring them up. But we haven’t lost her." Mary wants people to remember Mandy as she really was. There have been years of silence while court proceedings trundled interminably on. Two days after my visit, Mary says on the phone that telling the truth about Mandy has finally liberated her. She feels ready to move on and face the future. "It’s the first time I’ve felt like that in four years," she says. "For the sake of the girls we will leave the trauma behind. We will move on from here - and we will take Mandy with us."