Regrets? He's had a few, but then again, they're for another interview. At least that's what Marti Pellow says when we meet the morning after a triumphant gig at London's Koko venue, where 1,400 Wet Wet Wet fans turned out to see the Scottish four-piece on the comeback trail once again.

He is buzzing after it; it can be easy to forget how much your band means to people when you’ve been away for so long. “We went up there and sang a wad of songs, people had a great night and we all left with smiles on our face,” he says, beaming. “Job’s a good ’un!”

Pellow has led a chequered life. He was plucked from the dole queue to join Wet Wet Wet in 1982, and the band conquered the charts in the mid-to-late 1990s with their brand of soft-rock (or “sophisti-pop”, as Wikipedia cheesily categorises it). Pellow’s subsequent spiral into alcohol and drug addiction in his early 30s was followed by a career renaissance on the stage, with roles in West End and Broadway musicals. Regrets? Yeah. The man who once described heroin as “the ultimate painkiller” probably has a couple. But not today.

In any case, we're not here today to dig for dirt on the Scot's private life: now 50, lean and tanned, he is no longer the long- maned hunk from the Goodnight Girl video (which still gets breahtless YouTube comments such as "Marti Pellow sings like a sexy angel, I love his long hair, Marti is gorgeous").

He clocked up his half-century in March, but it was no big deal; he was performing in Willy Russell's Blood Brothers in Hastings on the day. "Was there a time that I thought I wouldn't reach 50? Yeah, probably around 49½," he jokes. "There's not really much I can do about it. I'm a relatively healthy man, and hey, things are going great."

Pellow's career in stage musicals happened almost accidentally when he was offered the role of Billy Flynn in Chicago after his solo debut album, Smile, was released in 2001. Since then, roles in Chess, The Witches of Eastwick, Evita and Jeff Wayne's War of the Worlds have been in steady supply, alongside his recent role as The Narrator in the aforementioned Blood Brothers.

“I always take a wee pinch at myself and go, ‘Huh? When did that happen?’,” he says of his stage career. “I’m kind of blessed that I have good people who surround me who can make it happen. I wasn’t born with jazz hands – I certainly was not – but I kind of understand where my limits are and where I can go as a singer. And that’s the point of reference; music is music is music. It’s not a big leap for me – in my head, anyway.”

Never officially split

Wet Wet Wet formed in 1982 – when Pellow was just 17 – and the band have never officially broken up, although drummer Tommy Cunningham quit over a royalties dispute and Pellow followed suit to enter rehab in 1999. They have reformed for gigs sporadically since 2004, and next year they head back out on the road for a British and Irish arena tour.

The tour was prompted, the singer says, by the realisation that they had only toured three times in the past 15 years – as well as their label Universal's intention to release a remastered version of their world-beating album Picture This. It's been 20 years since that album's release, but Pellow's memories of the era are still vivid.

"We were really at the top of our game with that album, and through the whole duration we were having success with Love Is All Around," he says. "I think that shows in our songwriting; we took a lot of energy into that record. We'd had success before, but we were on a different plane then, and I think we were a little bit more together with our songwriting. As a body of work, it seemed to resonate with me – and it still does now. It's something that I'm hugely proud of."

Much of the album's string arrangements, including the aforementioned Love Is All Around, were done with the RTÉ Concert Orchestra and the inimitable Fiachra Trench, who has been a friend of Pellow's and the band's ever since. Their cover of The Troggs 1967 hit spent an exhaustive 15 weeks at number one on its release in 1994. Even today, it remains the tune that the band just can't get away from.

"Well, I think there's enough time gone by that I can be reflective about that song, and look at it for the wonderful pop song it is," says Pellow. "It was number one for an incredibly long time, and you're looking at it, going, 'Okay, first month, second month, third month, where are we going with this?' It's like eating chocolate every day. There were people who loved that song and then, because it was about for so long, a bit of negativity started to come with it. But for us, it was a wonderful song. I remember playing it to [Troggs frontman] Reg Presley, and him saying to us, 'I love what you've done with it'. That's a great compliment and it's something I hold and cherish."

There is no getting away from the fact that the music industry has changed quite a bit over the past 20 years. How do you avoid becoming a mere nostalgia trip for people? Is there still a place for the Wets?

“Listen, everybody accesses their music in a different way,” he says. “Are we relevant? Hey, look. We make music and if people subscribe to that and like it, then great. Whether or not I lift up a paper and it says ‘Mass critical acclaim for Wet Wet Wet’ is not important, because the next day, the same page could say ‘Mass flop for Wet Wet Wet’. It’s not what’ll define us as musicians or individuals. We’re not coming out and singing novelty songs; they’re songs that still resonate with us as songwriters.

“Most importantly, hopefully the thousands of people who’ll come along will be able to tap into that as well.”

Since coming together again with intent, the door is open for another Wet Wet Wet album, too. They’re throwing themselves into studio sessions over the summer, “for our own peace of mind, and for the fans to see, ‘Hey, look, here’s where we are musically as 50-year-old men, good, bad or indifferent’.”

The rest of Pellow's year will be taken up by further appearances in Blood Brothers, and his solo career remains a going concern (he released Boulevard of Life last year). Life is good at the moment.

So, no time for those aforementioned regrets, then? “Oh aye, there’s millions. Millions,” he scoffs, chuckling. “Of course I do; life’s life. Imagine walking through life with no regrets. That’s one lucky person. Or is it? I don’t think it is, because apparently you learn from those. So aye, I wouldn’t change a thing.”

• Wet Wet Wet play Dublin's 3 Arena on February 23rd, 2016; Marti Pellow plays the National Concert Hall with the RTÉ Concert Orchestra on August 14th