Evita, Grand Opera House, York, June 9-14

The invitation to play Che, a character with rather more than a hint of Argentine Marxist revolutionary Che Guevara, came via the musical's lyricist Tim Rice, as Marti explains.

"The reason I got to be in the show is indirectly through his daughter who was a big fan of Wet Wet Wet, and to be thought of as the right person for Che was a great feeling. I was made up when he wanted me to do it."

Marti, 48, had already appeared in such musicals as The Witches Of Eastwick, Chess, Chicago, Blood Brothers and Jekyll & Hyde, The Musical and he was thrilled to join Bill Kenwright and Bob Tomson's touring production of Andrew Lloyd Webber and Tim Rice's most ;political musical.

Telling the story of Eva Peron, wife of former Argentine dictator Juan Peron, as she rises from humble beginnings to extraordinary wealth, power and iconic status as the “spiritual leader of the nation”, the show is supercharged with such hits as Don’t Cry For Me Argentina, On This Night Of A Thousand Stars, You Must Love Me and Another Suitcase In Another Hall, not forgetting Pellow's big number Oh What A Circus and his Act Two ballad High Flying Adored.

"I said, 'What were you eating when you wrote this,Tim?' , and he said, 'I don't know, but I'd like another plate of it!," recalls Marti, whose character Che is like a one-man Greek chorus, who reflects the voice of the Argentine people by stirring up conflict.

His performance is marked by a haunting sense of presence, always watching on, or singing with menacing poise. "What I learned from playing the Narrator in Blood Brothers is the importance of stillness, where you don't 'disappear' but you do nothing as you stand there," says Marti. "Your stillness is the thing, but you're raging inside, and suddenly the audience's eyes will drift to you. You're bringing them in, so when you then sing something it has great gravitas."

Analysing Che's role, Marti says: "He's the Greek chorus figure, the catalyst, the conscience of the Argentinian people. There's a triangle between Eva Peron, her husband and Che, and both of them are madly in love with her. She's doing what he wants her to do, commanding her audience, and all I do is observe!"

You can sense Marti's pleasure in playing a role that connects him with Che Guevara. "He's one of the most iconic images of the 20th century; Ernesto smoking his cigar in that Italian photographer's portrait or the young man in the T-shirt in The Motorcycle Diaries film," he says. "His grandmother was Irish, from Galway, and his father always said 'the blood of an Irish rebel ran through my son'. At the same time, he was such an educated man, he could have been a doctor."

Marti's Che constantly emerges from the shadows. "Here he's an iconic figure who's very dark in this piece but has his vulnerable moments too," he says. "That's how I like to play him."

The big moment for Marti comes as early as the show's third number. "Oh What A Circus goes from one to ten in three verses and suddenly there's a moment when it blows the doors off and you think, 'what happened there?!'." he says. "But then Che gets such a beautiful ballad in High Flying Adored".

Marti had shown his vocal versatility in Wet Wet Wet and in his myriad musical roles too, and he did so once more on last year's solo album, Hope, a collection of 16 songs from such Broadway and West End shows as Sunset Boulevard, Jekyll & Hyde: The Musical, Guys And Dolls, The King And I, A Little Night Music, Blood Brothers, Love Never Dies and South Pacific.

"From that album, I did a little show last year called The Hope Tour, which was my very stark interpretation of songs I enjoyed from musical theatre," he says. "There wasn't a bad seat in the house in a show like that, the way it was lit, when I came into the room."

He picked the songs with musical theatre impresario Bill Kenwright. "Hope was the name of Bill's mother, who was a great fan of musical theatre, as I was," Marti says. "I'd been doing musical theatre shows for ten years, but I wanted to do the Hope show in the way the songs were first written, to show their simplicity, which makes the lyric stand out more.

"So I did it with a pianist, guitarist, vibes, harp, double bass, two keyboard players, but no drums, and I was very relaxed in leisure wear on stage. It was very important that all I did was bring in old art-deco film lights to show off the performance space; the behind-the-scenes that you don't normally see. Going on stage with just a single beam of light, it takes 30 years to understand you can do that; I couldn't have done that at 21."

The unadorned presentation was in keeping with Marti's interpretation of songs by the likes of Brecht and Sondheim. "A good song is a good song, but when you break it down and show the shadows, it's starker, and there lies the complexity," he says.

Marti will be taking a more conventional approach when he returns to the Wet Wet Wet songbook at the Scots' post-racing concert at York Racecourse on July 26. "Damn those Marx Brothers, eh!", he says in passing reference to the film A Day At The Races.

"I did Newmarket Racecourse a few years ago, and Wet Wet Wet had just finished our arena tour at Christmas when the promoters who run these racecourse concerts said, 'How about doing six concerts this summer?'. I thought, 'nae bother', so we'll pull out all the big guns, and we're the type of band where the crowd knows the back catalogue and we and they can go down memory lane."

Will Marti watch the afternoon's race card on Knavesmire? "I kind of like a wee punt now and then, so I'll probably place a bet," he says. "Bang goes our fee that day. They better not pay us in cash!"

Evita runs at Grand Opera House, York, from Monday to Saturday, 7.30pm, plus 2.30pm, Wednesday and Saturday. Box office: 0844 871 3024 or at atgtickets.com/york. All-inclusive race day/Wet Wet Wet tickets for July 26 at York Racecourse are on sale on 01904 620911 or at yorkracecourse.co.uk

Please note: The role of Juan Peron is now played by Andrew C Wadsworth.