For a man with a pretty hectic schedule, Marti Pellow looks pretty relaxed in his casual threads in a comfy chair in an upmarket Glasgow hotel.

Perhaps he's had practice.

The chap still best known as the perma-grinned singer with Clydebank pop-soul combo Wet Wet Wet celebrated his 48th birthday on Saturday, by playing a concert at the Lowry in Salford. His show arrives at Glasgow's King's Theatre tonight on a tour, like the recording of the album it promotes, squeezed in between musical theatre engagements. Pellow has just finished a long run in Willy Russell's perennial crowd-pleaser Blood Brothers and goes into rehearsals for Evita immediately after his own 12-date tour.

With roles in Chicago and Jekyll and Hyde already in his stage portfolio, Pellow is more than entitled to be making an album that draws on the music theatre repertoire. That disc, Hope, comes out in the midst of a wave of such releases, with those of Susan Boyle and Girls Aloud's Kimberley Walsh enjoying the highest profile, but it belongs in better company. He may have come to his new career thanks to his pop fame, and insist that the jig is far from up with his colleagues in the Wets, but Pellow is knowledgeable and still eager to learn more about the vast repertoire of show tunes from mentors that include composer Leslie Bricusse, lyricist Don Black and, crucially, promoter Bill Kenwright.

"For the past ten years I've been stopped at the stage door and asked about making albums of the songs I was singing, so of course I have thought about how I would treat those songs," he says. "I'd finished Jekyll and Hyde for Bill and Leslie, and was going on to do Blood Brothers and I thought, 'Yes, I'm ready for that. I'll have a shot at it.' I thought I'd take two or three weeks to do it and then sell it in theatre foyers."

If Hope became a little more than such a seemingly throwaway idea, it is part of its success that it did not become too big a project. The timescale stayed much the same, and the instrumentation for the recording minimal, with Pellow's long-term musical associate Grant Mitchell supplying the piano accompaniment as well as a few other instruments and samples. Outside input did come from Kenwright, however, in discussion of the selection of songs, and it is via the impressario's own label that the disc has been released, named after – and dedicated to – his mother, who died last year.

"I wanted to do the album in a way that was quite stark, in a stripped-down way so you could see the structure of the songs. It is very easy to over-orchestrate and then you lose the essence of the song, although I suppose this approach was quite brave," says Pellow.

It also reveals a confidence in your own abilities. Without a doubt, Pellow's Hope cost a deal less to record than either Subo or Walsh's "stage" albums, and equally certainly is that what you hear is an accurate version of Pellow's voice, much as it will be on stage at the King's tonight.

"Grant and I can skate anywhere. I hope I challenge him, and he has a wonderful musicality and is a great pianist. I have always liked to record as live as possible," says Pellow. "I've done the big orchestra thing, the full bhoona, French horns galore, and it can be hard to find a place for yourself. So this tour will just be Grant with a small group."

Between his own discs and those with Wet Wet Wet, the singer has recorded in some of the greatest studios in the world, from Ardent in Memphis to Abbey Road. Hope was made at La Fabrique in San Remy de Provence, an old school venue with vintage valve equipment and a remarkable library of classical music on vinyl. It's clear that Pellow has a taste for such values, but adds: "It makes no difference whether you are in New York or Maryhill, once the door shuts -"

However, a major consideration in the choice of studio was that it was handy for home. Famously guarded about his private life and relationship, Pellow is quite unguarded and open about the fact his long-term partner Eileen Catterson is at their house in France on the day we speak. It has taken over as the couple's main abode, from a house in Brighton which once belonged to Laurence Olivier and had been lovingly restored by Blue Peter producer Biddy Baxter. After a few minutes of this personal detail, however, the singer abruptly cuts himself off from the topic with a joke about doing OK for a chap "from a small fishing village on the Clyde", and how he has always been a "blue sky boy".

We are on much more secure, and common, ground discussing the material on the album. As a stage door souvenir, its weaknesses are its strengths, because it may not actually contain a single number that Pellow has sung on stage. Willy Russell's Tell Me It's Not True is not one of his songs in Blood Brothers, but, he points out, he has heard it sung 400 times. The same applies to Once Upon a Dream from Jekyll and Hyde, although the total of overhearings is probably lower. Although they share disc space with Lloyd Webber (Sunset Boulevard and Love Never Dies), there is also room for Sondheim (Somewhere, Send in the Clowns, and Finishing the Hat from Sunday in the Park with George) and Rogers and Hammerstein (I Have Dreamed from The King and I, and This Nearly Was Mine from South Pacific), as well as Frank Loesser's Luck Be A Lady (from Guys and Dolls) and Jacques Brel's If You Go Away (from Dusty). The breadth of material speaks of an interest not immediately obvious in recent releases from some other artists.

"Jukebox musicals are not my bag and neither is Glee, although it serves a purpose in turning people to the well these songs have sprung from – but I like songs that are specifically tailored for the piece. And I believe I'm blessed to have an audience that is sophisticated and willing to come with me to something like Sondheim's Finishing the Hat, which is like foie gras.

"That excites me, and coming from a pop perspective it is not what I know how to do, so it is a learning process. That's what I get from musical theatre – those jagged shapes and different symmetry. You can't second guess these cats, they're of a different time."

Pellow has also been assiduous in seeing new musicals, as the tracklisting shows. Maury Yeston's Unusual Way is from Nine, which was inspired by Fellini's 8 1/2, and which Pellow saw Antonio Banderas do in New York. I'd Rather Leave While I'm In Love is from The Boy From Oz, which Hugh Jackman was performing in New York during Pellow's Broadway run in Chicago.

Although he could hardly be accused of taking the easy option, Pellow insists that Hope is "a simple record".

"It's not got world domination or even chart success written on it, it's just another side of what I do as an artist, something that I've been lucky to do has come to me through circumstances and a chain of events."

Hope is out now on BK Records. Marti Pellow plays the King's Theatre, Glasgow, tonight.