IT'S been a whole 15 years since Simon Cowell manufactured one of the most successful musical groups of his career. No, not One Direction - Il Divo. The classical crossover group, comprised of a global melting pot of singing stars, has a mightily impressive collection of strings to their bow.

They have sold more than 30 million albums, including 160 gold and platinum albums in 33 countries, and they were awarded the artist of the decade prize at the Classic Brit Awards in 2011.

And, while the quartet, made up of Swiss tenor Urs Buhler, German-born Spanish baritone Carlos Marin, American tenor David Miller and French singer Sebastien Izambard, has never really gone away, their new album Timeless is a comeback of sorts.

It's their first release since parting ways with Cowell and it has allowed them to do things their way for the first time. Buhler says their contract with Syco finished officially last summer: "It just kind of came to a natural end. We had our own creative ideas, and we wanted to carry on with these. We're very proud that we've done so. I think, with Timeless, we've created one of the greatest things we've done in 15 years."

Buhler and his bandmates speak highly of Cowell, who was originally inspired to create a contemporary multi-national tenor vocal group, following in the footsteps of The Three Tenors, Luciano Pavarotti, Jose Carreras and Placido Domingo. But things trailed off in terms of interaction with the music mogul in recent years.

"He was very heavily involved in the first three or four studio albums, but then I don't know if it's due to his own career being incredibly busy, but his influence and his involvement became less and less," Buhler recounts. "And even in the last couple of discs we did with Syco, he was hardly involved. He basically listened to the finished product." Buhler now describes Cowell as "just an old friend" who they are on "very good terms with".

The group is bubbling over with excitement ahead of the release of their seventh album, which also tidily represents their 15th anniversary. They say it is the "album we've always wanted to make", and the foursome are notably ardent when talking about how it was created without "anyone having any say or input".

Timeless, they say, was based on a "concept", and covers different genres of music from across the decades, right back to the 1930s. The record includes covers of songs such as Love Me Tender, What A Wonderful World and Smile, each of which was handpicked by themselves. "We've taken a song from each decade, the most beautiful song we could find, and we compiled all these together on the new album," Buhler explains.

One of Il Divo's favourite tracks on the album is Hola, a Spanish-language cover of Adele's mega-hit Hello, complete with a fresh orchestral arrangement. Miller points out that, while Adele has not directly aired her views over the track, she did grant them permission to cover it.

"Clearly she was pleased enough with our demo that we sent her, and our intentions with what we wanted to do with it, and she was pleased enough with the Spanish translation," he adds.

And would they ever like the chance to perform Hola with Adele? Izambard quips, sharp as a tack: "I'd have to think about that..."

As the four singers fall about in laughter for what feels like the 37th time in a few short minutes, it becomes wonderfully apparent that their humour is the key to their longevity.

Fifteen years in one group is quite the achievement, especially considering the Frankenstein-esque manner the already established singers, now aged between 45 and 49, were put together, and that many acts struggle to go the distance. Miller is adamant that their "really good sense of humour" is what gets them by when the pressures of travelling the globe wear them down.

"When touring, you spend time on planes, you get dehydrated, you're always running the risk of getting sick, you're in different time zones, you have jet lag... it's just the nature of the beast," he sighs.

"And I think that the four of us really use our good natures and our senses of humour to try and off-set that as much as we can.

"We try to keep things light-hearted, which is not always easy because, not only do we have the pressures of needing to pull off a show every night or every other night, but we're now the captains of the ship at this point. We are the producers, we are the directors, we are the owners and the people who shape the direction as well. We do what we can, we take it one day at a time, one moment at a time, one hurdle at a time and one performance at a time."

He jokes: "It kinda works... we're 15 years in, we're still alive, and we haven't killed each other!"

It hasn't always been this easy for Il Divo, though, particularly in the early days. Marin reveals the biggest obstacle they had to overcome.

"It was funny," he declares through his thick accent, adding: "I was born in Germany and I'm Spanish, but I was speaking more German and my English was terrible... And we didn't know each other. The language barrier was the most inconvenient part for me."

"In many bands, the differences between us would tear everybody apart and people would want to leave and we actually had that at the very beginning, it was difficult," Izambard chips in.

"We are just like a functional marriage, it's something that just works, although at the beginning it was difficult."

But through the barrier and the tough times they made it, unscathed, and it's obvious that even after years of world tours, Guinness World Record coups - in 2006 they were named the most successful classical crossover group in history - and performing for the Queen, they are as tight as ever.

There does not appear to be an end in sight for the group, either, who insist they have at least another 15 years in them as a group - as long as their fans keep on listening.

Izambard insists: "It would only stop if the fans were to decide that they don't like our music." So here's to another 15...

* Il Divo's new album Timeless is out on August 10.