The unbelievable coincidence behind the new series of Long Lost Family

That search led to them writing in to Long Lost Family at almost exactly the same time. Only detective work by the show’s researchers – and a subsequent DNA test - uncovered the fact that they were looking for the same woman and they were half-siblings.

It’s a scenario that left not just them but both the show’s presenters Davina McCall and Nicky Campbell reeling. “What are the chances of that happening?” McCall asks now. “Not only that they applied at the same time but that the same researcher read both application forms and saw the similarities? Without that they would never have done the DNA test and they would never have found out that they were looking for the same person.”

The new siblings’ reunion proves joyful, yet ultimately bittersweet. While researchers did manage to trace their mother she didn’t want to be put back in touch - the pain of reconciliation apparently greater than the pain of separation. “It’s devastating for them, but they respect her choice,” says Campbell.

That news is delivered off-camera, and isn’t addressed in the programme, although the cameras are rolling when the siblings meet. As always, the two presenters absent themselves from this final act – a tear-shedding moment observed without intervention.

They don’t catch up with it until months later in an early preview of the programme. “We are actually watching it in the way the viewers sees it, except we know them,” says McCall. Campbell, who said a link to the programme was “burning a hole in his iPad” all weekend, admits to being “in bits” by the end of the opening titles. “I texted Davina and Sally, the producer. It's astonishing, isn't it? You see a four-minute reunion; you see a microcosm of a lifetime's pain.”

Is the search for identity the theme that underpins the show?

Davina McCall That and belonging, finding that familial bond. I think I had that when I had kids: suddenly I look at them and I go, “That's me, that's a part of me.”

Nicky Campbell I think that's why this show works - because you're not just watching these individual stories, you're watching every person's story. This show is about all of us, about the human experience. So it’s about so many things, identity, and belonging, and redemption too.

Which scenarios touch you most?

DM I think it's probably what we identify with, so if there's ever anything about a mother-daughter relationship, I'm always in tatters. Also because I've lost my sister (McCall’s half-sister Caroline passed away aged 50 in 2012 after battling cancer) there's been occasions when sisters get reunited and that always really affects me. Obviously, I'm not naive and I do understand that you can introduce people and they don't necessarily hit it off, but I still think: wouldn't it be incredible to discover a half-sibling just out of the blue?

NC The ones with a sense of redemption affect me most. They’re the most raw and the most difficult ones, but the most life-affirming too. You realise that it’s never too late.

DM One thing I am really pleased about is that we’ve done a lot of these programmes and you could worry about compassion fatigue but this show just keeps talking to me in a way that nothing else does, it taps into something deep within me and that's not abated.

Do you have an innate sense of which stories are going to be the most powerful?

DM All of them are powerful, because you know that the people who have written in have tried everything, and they are desperate. You never forget that. But sometimes with stories you think, “OK, I know how this works.” And then you get there and it's always different, it's never what you think it's going to be.

NC What I find interesting is that people’s emotions change even within the course of one conversation. You see it a lot with the people who are being looked for: they start out being brave and it’s, “I'm interested in it, but it's not a particularly emotional thing” and then they realise as they talk that it really is. Sometimes they’re speaking about this for the first time in their life. They’ve not allowed themselves to think about stuff but the minute you do, it all comes out. It’s fascinating to watch.

For all the stories that we see on-screen, there must be many we don’t.

NC So many people aren't found.

DM The ratio is something like one in six in terms of success. And quite often people get found, but they don't want it to be on TV, which is understandable.

And obviously some people don’t want to be found?

DM It happens all the time….

NC Sometimes it's too much for people to revisit the past, it's just too much of a volcano.

DM It's been suppressed for so many years they can’t go back. But obviously that is very hard for the person searching. That’s something we deal with off-air.

NC Davina is amazing at bringing news. Sometimes she'll have bad news to talk through off-camera, and sometimes she's bringing good news, but she's brilliant at helping people through. My role is more finding out what the story is and exploring that.

DM We also have some amazing, compassionate people on this show who sometimes have to do a really tough job. We had one woman who was 74 whose baby boy was taken out of her hands at four weeks old and taken away by social workers. She had been looking for him every day of her life and one of our producers had to go and see her to tell her we had found out that he died at 12 weeks old. Can you imagine?

Has working on the show made you think differently about adoption?

NC It’s complex. On one side you have people saying it’s too slow but then you get a contrary message that babies are being taken away too soon. I think there are necessary checks, and we are doing our best. Since I went into working with adoption charities [Campbell himself was adopted at four days old], I have changed my view of interracial adoption: originally, I used to think love is love, whatever colour or race the adoptive parents are. But when you're adopted, there is a lot of identity crisis anyway. I had a big identity crisis because I had blue eyes and no one else in my family had blue eyes. So if you are adopted, plus you come from a different culture, plus you look completely different, it's like a triple whammy. There are situations where that doesn't matter, but in the long term it's an extra layer of identity problems.

DM For me I’ve realised there are huge dangers around social media when it comes to adoption. The worst thing you can do if you are adopted or searching for someone is to just go and look them up on Facebook, because Facebook doesn't always tell the truth and unless you're very robust it can end up being very painful and very hurtful and you might get rejected. So go through a mediator, or go through somebody that can help you. You know what it was like Nicky – you did it all on your own, and you were 29.

NC And it’s very difficult, not just the build up to it, the reunion, but dealing with that afterwards. Sometimes it creates even more of a “Who am I?’’ feeling than there was before.