But everything comes back to the fact that their eldest child isn't there any more.

I met Kate and Gerry yesterday, a week before the third anniversary of the fateful day Madeleine disappeared from their holiday apartment in Praia da Luz, Portugal - May 3, 2007 - and a fortnight before her birthday on May 12th.

Kate told me: "Her birthday is actually much more difficult than May 3rd.

"It is a day when we should be celebrating Madeleine - and celebrating WITH Madeleine."

Kate is bone thin and looks very fragile and, while Gerry might appear to be coping well on the outside, you can clearly see the pain in his eyes.

They both have a haunted, strained look but somehow they manage to get through the day.

The twins, young as they are, know that things have changed but also that they are part of a family that has a world of well-wishers.

Kate, a GP, said that one of Sean's playmates had asked her "Are you a doctor?"

She recalled: "Sean just came in and said, 'Mummy was a doctor but her job now is to find Madeleine'. He was straight in there. So they understand that we have got a lot of support."

Gerry, 41, and Kate, 42, draw comfort from that. But even while enjoying a family outing and being able to laugh with the twins, Madeleine's disappearance casts a shadow.

Kate said: "We had a lovely day last week and it was really sunny and you could smell the grass being cut and I thought, oh, this is really nice. And then it just kind of gets you.

"Madeleine is still not here but we do have periods of normality. In fact, I would say it's just changed, in that it is a different kind of normality now.

"You still have to do the cooking and washing, we've got Sean and Amelie and we have lots of time with them. And we go on trips, they go swimming."

As well as having to put on a brave face for their children, they both find themselves feeling almost guilty for any moments of snatched happiness.

As Kate put it: "Suddenly you realise it's actually tinged."

Gerry has gone back to work, but that was tough to start with.

He said: "It was a little bit awkward at the beginning but at that time I found it much easier when I am mentally active, both from a campaign point of view and workwise as well.

"To be honest, most people were just really glad to see me. When I went back to work, quite often Madeleine would be there on the front pages of newspapers. So it was a bit awkward but it's not been a problem."

The twins are now old enough to know what happened to their big sister. But Kate and Gerry have had to be careful about being honest with them about Madeleine, not frightening them.

Initially, when the twins asked where Madeleine was, Kate would say she was lost.

But as they grew older they started to ask more questions. Kate said: "I think it was last year Amelie said to me, 'Has Madeleine run away?'

"And she kept asking me in a public place so it was a bit tricky at first, and she said, 'Because it's not nice to run away'.

"That really upset me because I thought I don't want her to think that Madeleine is at fault. So, probably about the third time she asked when we were at home we just explained that someone had taken Madeleine.

"But we tried to make them understand in as gentle a way as possible. It's a bit like stealing, you know.

That's how they understand it. So they know someone has taken her and they know it is wrong."

It's hard to imagine how tough it must have been to tell two little children that nightmares can happen and kids can be abducted from their beds.

Gerry said: "They believe it was a man that took her. It was a naughty man and we need to try and find them. So part of what they say is that mummy is working to try and help find Madeleine."

Kate added: "They constantly spot things like a car sticker or a luggage tag or a wrist band and they point it out and say, 'Look, they are helping us too'."

From the moment Madeleine disappeared, Kate and Gerry, who both appeared on my GMTV sofa today to talk about their daughter, have moved heaven and earth to try to find her.

Their campaign, which has included a high profile visit to the Pope, a TV and poster campaign and appearances on the likes of the Oprah Winfrey show, certainly got the message across and Madeleine's name was known worldwide.

But there has been a dark side, too.

Kate and Gerry found themselves accused of being neglectful parents and even complicit in Madeleine's disappearance.

They were vilified for leaving her in their holiday apartment while they dined nearby with friends.

They were even criticised for "coping too well" because they didn't break down in front of the cameras, as if this meant their grief wasn't genuine.

Anyone who sees them in private knows this to be horribly unfair and completely untrue. Their grief is deep, raw and almost impossible to imagine.

Gerry told me: "You get criticised whatever you do from some quarter. What you need to do is make decisions for the right reason and do it with the best intentions.

"Ultimately, we make our own decisions. But I think probably, more than anything, I'd say if we could turn back the clock and change what happened then obviously we wouldn't have done it.

"What I would say is people have got to put themselves in our position and ask what would you do if it was your daughter?"

Those who continue to condemn the couple surely accept that they have paid a truly horrendous price.

Under Portuguese law, Kate and Gerry found themselves as "arguidos" which translates roughly as suspects.

So not only did they have to cope with their child being missing, they had to endure being cross-examined by Portuguese police and having mud thrown at them.

Kate, in particular, found the vitriol tough to handle, especially in the early months after Madeleine's disappearance.

She said: "I wasn't expecting it because all I could see was our daughter has been taken and she is being subjected to something terrifying. But it is a small minority now."

In their ongoing campaign, Kate and Gerry would dearly love to see a full government review of the case.

Gerry said: "It is not right that an innocent, vulnerable British citizen is, essentially, given up on. We don't think it is right that, as parents, we have to drive the search.

"Of course, we will but not everyone has the same resources that we have had."

They have also produced a pack for Brits travelling abroad. It contains posters and car stickers with images of Madeleine as she was when she went missing and how she might look now.

Gerry said: "It is very much about keeping her image out there. Who knows who will end up seeing her. But if you don't have an image of her out there it is less likely."

I had to ask them whether they really thought that Madeleine was still alive.

Kate said: "Certainly, in my heart, I feel she is out there.

"There is nothing to say she isn't. So we carry on working and thinking like that."

Kate still has a clear image of her daughter in her head but it is the image of a child frozen in time at four years old - or, as she describes it, "Madeleine at four years minus nine days".

She said: "I can still hear her voice and we have video of her. Every so many months we sit down and watch that."

Gerry said: "A lot of this campaign stuff, it's almost like the abstract Madeleine. Our own video, it's ours. Sometimes you have got to embrace the grief. It's almost like you have to let that out."

For a pack and more information, go to findmadeleine.com