Kate McCann did something frighteningly normal the other day. She stopped at a petrol station, filled up, locked the car and went in to pay, leaving her eight-year-old twins, Sean and Amelie, strapped in the back seat. As she stood anxiously at the till, they pulled faces at her through the window.
''It was the first time in six years that I have been able to do that,'' she says. ''I was very conscious of what I was doing. I left them behind for a few moments. All the time they were in view. They used to protest when I took them inside with me to pay, though they knew it was, as Sean once said, 'because someone might take me.' ''
It is almost exactly six years since their older sister, Madeleine, vanished while on a family holiday in Portugal and became the focus of one of the most intense, prolonged and high-profile public campaigns ever mounted to find a missing person. In that time, and despite many investigations and false accusations - the most hellish being that the McCanns were themselves implicated in their daughter's disappearance - not a single piece of evidence has emerged to show that she is dead.
Kate and Gerry McCann, balancing realism and optimism, believe it is possible that their daughter, just a few days from her fourth birthday when she was abducted, will be found. How else would they go on?
''As there is nothing to suggest that Madeleine is not alive,'' Kate says, ''we have to keep looking for her. We all know there are cases of missing children, presumed dead, who have been found alive years, sometimes decades, later.''
Little by little, they have schooled themselves not to dwell on the lurid possibilities that tormented them in the early days - that their daughter had been snatched by a paedophile network or met with a cruel death - but they are never free of questions and sometimes they are ambushed by old fears.
''None of the scenarios are good when your child has been taken,'' she says. ''You're in a dark place some of the time. You get upset. You get angry. I have spent hours thinking of the possibilities. Do I want to know what happened? I've sat myself down and asked myself: if you knew and it was truly awful, would that help?''
Her familiar sharp-boned face looks weary. At 45 she is pretty but lines of anxiety show at her mouth and crease her forehead. She seems poised, not in a studied way but like a person suspended. Although she says she and Gerry are ''in a better place'' than at any time since Madeleine was taken, her sentences sound as though they are coming from a long way off.
''Living in limbo with this void and uncertainty is truly dreadful. It's hard to rest, to find peace. It's unsettling and uncomfortable all the time. Even on a 'good' day, that feeling is lurking.
''And, of course, you can never stop until you know; you're on a treadmill - you can't get off. It's draining. Until you know, there is no true peace. We need to know for us and we need to know for Sean and Amelie so that, god forbid, in another 10 years or so they don't have to cope with this distressing limbo, too.''
For the moment, Madeleine is as real to them, and to the whole family, as an absent person could be. They talk about her all the time, observe her birthdays with a party and gifts, give her Christmas presents and fill a keepsake box with things they think she would like - drawings, school work, sweets, a leaf. Kate fills Christmas stockings for three. ''There is part of me that has to do it.''
There are photographs of Madeleine everywhere. Her room at the family home in Rothley, Leicestershire, is as she left it when they went on holiday, plus the unopened presents.
Kate opens and closes the curtains twice a day and sometimes stays to absorb what she can of Madeleine. It is not what everyone would choose to do, she agrees, but it is her way. Sean and Amelie still share a bedroom. Soon there will be the practical issue of what to do with a ''spare'' room one of them may need. ''But that was the room she left and it would be familiar to her. It would be hard to dismantle it.'' Gerry, 44, a cardiologist, has a more practical turn of mind. ''So, in time, we may perhaps look at it differently.''
The twins' understanding of what happened in Praia da Luz on the night of May 3, 2007 is matter-of-fact and unafraid. ''We explain it like a burglary,'' Kate says. ''You must never take something that doesn't belong to you. Madeleine belonged to our family and someone, who had no right to her, took her away. We also explain that this is very rare. It doesn't happen every day.''
Touchingly, the parents' protectiveness and resolve is echoed by their children. When Sean was little more than a toddler, he reassured them: ''Me and Amelie will go on looking for Madeleine.''
At school, they talk about their missing sister to other people. ''I have to ask: 'Is this all right?' '' Kate says. ''Because I know some people like to pretend it didn't happen and the world's a lovely place.''
Throughout that disbelieving summer, the international news was dominated by a single crime and a single small face. Madeleine was snatched from her bed between regular checks made by her parents, who were dining with friends in a tapas bar 50 metres away from the apartment. They thought the arrangement was so normal, so ideal, that they never questioned it and plenty of sympathetic parents told them they would have done exactly the same. There were accusations of neglect, too, but no one was harder on the couple than they were themselves.
''We thought we had worked out the best plan,'' Kate says. ''It seems very different now. I have persecuted myself about that decision for years, even though deep down I knew I was a caring parent and how much I loved my children.''
In the early months, even years, she despised her daughter's unknown thief. ''The thought of her feeling fear and wanting and needing her mummy and daddy provoked so much pain. It still does, when I wander down that particular path.''
Kate is a practising Catholic and when asked about forgiveness she used to say she needed to understand the motive. Now, tentatively, she feels differently.
''I think I could probably forgive Madeleine's abductor whatever the circumstances. I don't know whether it's simply because I'm stronger, or because there's no benefit in not forgiving someone. I can't change anything and I don't want to be eaten up by hatred and bitterness. And maybe there is an element of pity - what kind of person could do something like this? Of course, forgiveness will always be easier if there is remorse.''
The Scotland Yard review of the case, set up two years ago, has brought the McCanns a degree of equanimity because it relieves them of the burden of maintaining the search through private investigators.
''Emotionally, it helps,'' Kate says. ''We were doing so much ourselves. Now at least it is not totally down to us. We have been able to switch off a little bit. If we go away, we know there is work going on. We are encouraged by what the Met team has done and found. They have uncovered so much.''
Thirty officers are working full-time on Operation Grange. ''As time goes on, it is hard to maintain the level of motivation but, if anything, they are more determined now. But we still want the Portuguese police to reopen the investigation [closed in July 2008]. We want to find our daughter and the person who committed this very serious crime. This case needs to be solved. Why would anyone not want to solve it?''
In the meantime, there is no let-up in the McCanns' fund-raising and awareness campaign Find Madeleine (whose 10th birthday is on May 12). Their all-consuming focus is, and has been, astonishing. Kate, a former GP, has become an ambassador for the charity Missing People, which supports the families of some of the 250,000 who go missing in Britain each year. A transition, maybe, from personal grief to a concern for people who lack her own professional acumen and whose bleak stories do not make the headlines. ''Before Madeleine went missing,'' she admits, ''I was horrifyingly ignorant about this issue myself. It really is much bigger than people realise.''
Once, Kate was afraid that the intensity of grief would threaten their marriage, because she could not bear to take pleasure or comfort in the physical side of their relationship. ''I'm pleased, and relieved, to say our relationship is really good,'' she says. ''Given that we've made it through so many awful things over the past six years - and not just made it through but are united, strong and very happy together - then we can make it through anything. We'll survive.''
Nor are the many reported sightings of Madeleine as upsetting as they used to be. ''I am able to rein in my emotions quite easily. [The reports] need to have real credibility. It is encouraging, though, six years on, that people are still looking and haven't forgotten about Madeleine - that, in itself, gives us hope.
''There are moments when you despair but they are infrequent now. As someone said: It's not that your burden gets any lighter. It's just that your legs get stronger. That really sums it up.''
The Daily Telegraph, London