Five years ago they were a bunch of first-year undergrads at the Royal Conservatoire of Scotland.

Last month they scooped top prize at the UK's most prestigious choral competition. "Les Sirenes won me over instantly," said Choir of the Year judge Mary King of the all-female group from Glasgow. "Their music went straight to my core." The choir's founder and conductor, Andrew Nunn, a slip of a 24-year-old RCS master's student with a broad Middlesbrough lilt, describes the win as "the pinnacle".

"I knew we had the potential," he grins over a cup of tea near the RCS, "but I couldn't be sure that all the elements would come together. This kind of thing is about beautiful singing but also about playing your cards well. We deployed Poulenc's Ave Verum Corpus and Elgar's The Snow to impress judges at stage level, but if we'd done such serious repertoire in the grand final, I don't think we would have won."

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At the final Nunn went for the heart, with arrangements of Soldier, Soldier and the Billy Joel song And So it Goes. "I reckon that clinched it," he says. "It was the simplest number in the final. Some of the choirs tried ticking technical boxes, but I wanted the girls to show off how beautifully they can sing a long legato line. I wanted to draw the audience in, to get a tear."

This is Nunn's seventh year as an RCS student. He arrived as an undergraduate oboist but soon turned his energy to vocal music and is now into his second master's (his first was in oboe and singing) in choral conducting. It's the first year the RCS has offered this specialism, and Nunn is the first student to trial it.

Les Sirenes (the name is lifted from the last movement of Debussy's Nocturnes) began life as an ad-hoc initiative: a few girls were putting together some choral pieces and needed a conductor; Nunn happened to be at hand. They had a laugh together and kept up their Thursday night rehearsals. Just for something to work towards, they entered Choir of the Year. "That time we didn't even get through to audition stage," says Nunn. "I've still got the recordings we sent in – they're awful! No blend, bad intonation - We were all learning on the job."

Back then it was a choir of eight; now there are 25 members. Anyone who's heard Les Sirenes in recent years tends to remark on their sound quality. "It's a choir of soloists," Nunn explains. "They're not yet opera singers – mostly they're undergraduates, still finding their voices and a world away from singing opera, but they all have operatic potential."

What he means is the timbre, the vibrato, the weight of their voices: "Which actually goes against choral singing. To make a choir of vocal soloists is difficult, but it gives us a unique sound. Full-bodied, rich – there are a lot of girls choirs and a lot of older ladies' choirs, but it's less common to hear the 18-30 age group singing this repertoire.

"I'm careful to arrange which singers go where. Lighter voices on top, heavier voices on the middle and bottom. I also choose repertoire that plays to their womanly sound: we don't bother with music that choir boys would do better."

Nunn speaks with bright-eyed authority: it's easy to see why his career is already gathering rapid steam. This summer he worked alongside Christopher Bell as student conductor with the National Youth Choir of Scotland; last year he was Gibson Fellow with Tim Dean and the RSNO Chorus. He conducts the Hutchesons' Choral Society, the Thomas Coats Memorial Choral Society, the junior choir at the East Glasgow Music School and the chamber choir at the RCS Juniors. On Sundays he trundles down to Carlisle to conduct a community choir – and all this on top of a master's programme.

He underlines his gratitude to Les Sirenes, pointing out that "the girls get no course credits for singing in this choir, so their commitment is amazing." Nunn himself doesn't get paid for his work with the choir – "not in monetary terms, but I've built my career thanks to the training I've had with them."

The support of the RCS has been "fantastic in recent years" he says, choosing his words carefully. "It takes time to build these things up. Now they give us access to rehearsal spaces and music from the library, and they paid our expenses to get to London for the grand final." Les Sirenes does make a rather nice piece of publicity for the institution, of course - "I suppose that by winning the competition we've paid them back," Nunn allows.

What's next, then? The choir is putting out feelers to record labels for a debut disc; in the meantime you can relive their triumphant Choir of the Year final on television (Friday, 7.30pm, BBC Four) and hear them live at their annual Christmas concert (Mackintosh Church, Queen's Cross, December 14).