THEY were in a car park of a Little Chef when their lives changed.

It was a Sunday afternoon in 1992 and Capercaillie were heading home after a gig in Nairn the night before. They pulled in to the grounds of the "quite strange" branch of the roadside diner somewhere near Stirling and the record company rep who was travelling with them jumped into a phone box. The UK charts had been announced and, after calling his office, he told them, "we've done it" – a UK top 40 hit. Their first – and a landmark for the Gaelic language.

"We were absolutely ecstatic," vocalist Karen Matheson remembers. "It was so exciting."

That hit was Coisich A Rùin, a reworking of a 400-year-old waulking song. With rich lyrics of hunting and sailing, of geese, seals and swans, it was like nothing else in the charts and like nothing many listeners had heard before. The band knew the breakthrough meant something to the musical community they moved in. "We were excited for the tradition as well," accordionist Donald Shaw says. "We were part of something."

Coisich A Rùin had first been released on Delirium, the Argyll band's breakthrough record. This year marked the 30th anniversary of long-player that influenced a generation of folk artists and remains a fan favourite – their defining release to many. When they play Celtic Connections in January in a special performance with the BBC Scottish Symphony Orchestra, the audience will likely hear at least one of its tracks. "We've released a dozen albums over the years so we have recorded a lot of material and some soundtracks, but quite a lot of the repertoire of Delirium has survived in one way or another in live performance. There's tracks there that are still part of what we do live," Shaw says.

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"At the time, we were just coming out of a record contract with an American record company and we'd been picked up by a London-based label, Survival, and they had quite big aspirations for us that I didn't think we really related to at that point. We still thought of ourselves as being a wee folk band from Argyll that was doing our thing."

Making the album, Matheson says, there was a sense that "everything was growing and developing". "It did feel like a pivotal thing," she says. "It was a defining moment for us – it felt like we were real musicians."

Those musicians included Mark Duff, Charlie McKerron, John Saich and Manus Lunny, whose brother Donal produced the album. There've been some line-up changes along the way and Capercaillie's members now include Matheson, Shaw, McKerron, Lunny, bassist Ewen Vernal, Michael McGoldrick on flute and whistles, percussionist David Robertson and drummer Che Beresford.

Despite the multi-member line-up, Matheson was often the sole focus of media attention and became the face of both Capercaillie and the poster girl for Gaeldom. It's not something she enjoyed. "I would get quite annoyed that I would be at a photoshoot and they would be at the pub," she recalls. "I used to think, 'why are they getting to have all the fun when I have to do this?'"

Then there was the "pressure and responsibility" of being held up as a spokesperson for Gaelic. But amongst all that, Matheson, husband Shaw and their bandmates were "having a ball", doing "crazy things" like driving from England's south to Lewis and back again within 48 hours to meet an increasingly busy schedule of gigs and commitments. "It was a buzz," she says.

Throughout their careers, Matheson and Shaw have been clear that Capercaillie is a group effort. When Matheson was honoured with an MBE in 2004, she said she was accepting it on behalf of every member and the pair cite Duff and Saich's broad listening tastes as an important influence on their sound, a sound that became increasingly confident during a fortnight hanging out in a home studio, writing in preparation for the Delirium sessions. That was on the suggestion of Survival Records and gave them, Shaw says, "the opportunity to experiment". By the time they got into the recording studio with Donal Lunny, whose credits go from Kate Bush to Elvis Costello to Baaba Maal, they'd something special on their hands. "It was the first time we'd really worked with grooves and a drummer," Shaw says. "Because we didn't know the rules around that we were really kind of finding our way and creating tracks that weren't so conventional."

It wasn't the band's first release – there were three studio albums before Delirium, all released in the 1980s. But in the 1990s, the band were on-song, beginning an international career that's seen them sell more than a million albums, rack up a raft of silver and gold records, create the soundtrack for Liam Neeson's Rob Roy movie and gig in more than 30 nations from the US to Iraq. It's been a journey not just for them, but for Scottish traditional music.

That journey coincided with a renewal of mainstream interest in celtic cultures. Runrig had already had chart success with Loch Lomond and their Heart Hammer EP made it to a chart position of 25 in 1992. But it wasn't until 1995 that An Ubhal as Airde would give them their first Gaelic-language hit. Meanwhile, Ireland was about to embark upon the Eurovision-winning streak that would bring Riverdance to the masses.

The National: Capercaillie singer Karen MathesonCapercaillie singer Karen Matheson

And away from the TV cameras, Fèisean nan Gàidheal was founded.Working to increase participation in traditional Gaelic arts in communities across Scotland, the independent umbrella association of the fèis movement helps around 6000 young people take tuition every year and delivers a range of cultural events.

"As a band, we came to the fore at the same time as Runrig," says Shaw. "We were aware of incredible community that they built up and one thing that we came to realise through touring internationally was that the Gaelic songs, they spoke to people. These songs have such a strong magnetic pull in terms of their melodic power, their rhythms, their stories of travel and immigration and loss and love and working ways. I think we'd probably leaned on the power of these songs to try and create a story of who we were and what we were about.

"And itt's important to remember that when we started, other than a band like Runrig, there wasn't an overriding sense of respect of the tradition that there is now. Thirty-five years ago the only trad music you might hear would be pretty much kitsch Hogmanay stuff or the Tattoo. That's changed through the work of the fèisean movement and others. We have been part of a renaissance of bands and music but that's helped with the general problem about people knowing the value of their own tradition.

"Karen and I back in high school would have not been particularly up-front about the fact that we were in a folk band – 'I'm not going to tell people I'm going to school with my accordion'. You can now stand outside school gates and see a clarsach or a fiddle going in. That's beautiful for us to see, as musicians."

The secondary Shaw refers to is Oban High, where the band formed. They started out in local halls before expanding their fanbase outwith Argyll, across the west coast and islands and ultimately across borders. Fan Sean Connery famously said Matheson's throat was "surely touched by the hand of God" and they celebrated their 30th anniversary year in 2013 with At the Heart of It All, which brought in a cast of guest artists including Kris Drever and Julie Fowlis – big draws in their own right and names often featured on the bill of Celtic Connections.

Shaw is now creative producer of the annual Glasgow festival, having stepped down from the post of artistic director in 2018.

Anoushka Shankar and the Scottish Chamber Orchestra will open this year's event, which marks a return to live performance following the long shutdown caused by the pandemic. The programme also includes a tribute to Jackie Leven, a performance by Kathryn Joseph and the Tinderbox Collective and an acoustic night featuring James Grant, Bernard Butler and Norman Blake of Teenage Fanclub together.

As always, there's a strong international element, as with the work Matheson and Shaw have done away from Capercaillie. Both have worked with artists from different traditions, such as Algerian singer ‘Idir’, Breton guitarist Dan Ar Braz and Portuguese Fado star Dulce Pontes for Matheson. For Shaw, credits include turns with Peter Gabriel, Ornette Coleman and Bonnie Raitt.

"If Celtic Connections has managed anything, it is to shine a light on our own tradition through music from the rest of the world," Shaw explains. "It's that thing of if you're in your own space and you're not seeing what else is out there, you never really understand the true value of what you do. Over the years the festival has tried to position Scottish traditional music as a form of world music. We are saying when we bring in the likes of Anoushka Shankar or Youssou N'dour, we are on the same pedestal as these stars. We need to celebrate that our music lives in that same space.

"The festival is successful because it's concentrated more on the word 'connections' than 'celtic'."

The event is also renowned for its atmosphere. Matheson and Shaw can't wait. They played the Wickham Festival in August, their first in-real-life performance since the outset of lockdown. "It was amazing just being with each other again and performing live on stage," Matheson says. "I don't think you can replicate that. Online is great fun, but the thing that you really begin to understand is your performance is influenced by the audience reaction. You think it's the same without that, but it's not the same; you grow with the gig in response to them. It's a two-way process which has been sorely missed."

"What drives all musicians is feeling the love in the room," Shaw adds. "That probably is the crux of why people create art; it's because they want to be loved, probably. You want to feel that back and you can't really swap that for a screen. Musicians in the last year have felt very grateful for the support they've had with events going digital, which has been a bit of a safety net and kept us going, but the live environment is not just about the idea of playing a show and doing a job and making money, there's a genuine sense of wellbeing. For some people, it's a lifeline.

"Those few seconds of silence after the applause of walking on stage and before the first few notes, it's magical – you can't bottle that."

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Live recordings capture at least some of the essence Shaw describes. Capercallie Live in Concert, from 2002, captures a set including Rob Roy Reels and, yes, tracks from Delirium. Waiting for the Wheel to Turn is not on the live album, but remains one of Delirium's most biting and overtly political tracks.

On it, Matheson sings of a new Clearances in rural communities amidst a spate of property-purchases by strangers who sit "like a king with his gold from the south". "I was quite a young angry man, complaining about everything," says its writer Shaw. "I was 20.

"I don't backtrack from a word of those lyrics, they still hold true. But I probably used our southern neighbours as a target because that also played into my kind of pro-independence feelings even back then. Now I'd probably be a little bit more accepting of the fact that it's not got anything to do with nationality, it's a problem of social sustainability and wellbeing and community," he goes on, referring to the growth in second home ownership in the Highlands and islands by people from Scotland's cities too.

The process that sees "waves of wealth/Washing away the soul from the land" is also playing out more broadly across other parts of the UK, he says. "The lyrics of that song are almost appropriate again, but it's not just about Argyll any more, it's about people in Cornwall or Wales seeing the same thing where the local way they were brought up, unless they inherit their parents' house there's no way for them to live because we have put such a high premium on tourism as an industry in the UK generally. It's still relevant."

Trad songs, Matheson points out, are often political. "That was just the first time we'd started writing about these subjects, these age-old problems," she says. "It's just the same old story unfolding of inequality and basic human rights."