KT TUNSTALL was playing with her mate Kevin Cormack, aka Half Cousin, when she got the call in 2004. “We were on tour with the Manchester band The Earlies, about 20 of us on a tour bus having a great time,” she recalls 18 years later. When the summons came, she had to rush back to London, missing a couple of gigs.

“I got the call because Nas the rapper pulled out which was just hilariously weird. The guys in The Earlies didn’t really know I was doing my own thing. I was keeping quite quiet about it. And so when I went back out on the tour, we were in Fibbers in York, just a tiny little … I dunno, 300 capacity? And I said to the barman, ‘Can we turn the little TV on above the bar because I’m going to be on TV.’

“And the band were like, ‘What do you mean you’re going to be on TV? What the f*** are you talking about?’

Still, they all huddled around “this tiny little television” and watched. And in front of their eyes Tunstall transformed from a jobbing musician to a star.

Such was the impact of her first appearance on BBC2’s Later, performing Black Horse and the Cherry Tree. A stripped-back performance, just Tunstall, her guitar and a loop pedal.

“My eureka moment was I had never seen anyone bash their guitar to get the drum beat. And we finagled the tech set-up so I could put my vocals and my guitar onto the same loop.”

The camera turned and three minutes later Tunstall’s career was on the rise. Such was the impact of her performance.

“I still find it quite difficult to watch,” Tunstall admits from her home in Los Angeles, “because I still have part of me inside thinking, ‘I’m going to mess it up.’”

The journey from Fibbers in York to LA started with Tunstall’s appearance on BBC2’s music show Later. And the Scottish singer-songwriter is not the only musician whose career was transformed after appearing on the show. The likes of Seasick Steve and Ed Sheeran can also point to their appearance on the BBC music show hosted by Jools Holland as a significant signal boost for their career.

For 30 years, Later has been the first port of call for music on television. Admittedly, for much of that time, it has been one of the only ports of call for music on television. But three decades is something to celebrate.

Tonight BBC2 does just that. Jools’s 30th Birthday Bash marks the anniversary of the first show (actually broadcast on October 8, 1992; there have been more than 450 shows since), with performances from Robert Plant, First Aid Kit, the aforementioned Seasick Steve, Eliza Carthy and Michael Kiwanuka and more. In short, the kind of eclectic line-up that has been a mark of the show since its inception.

“The diversity of the show came from a desire to genuinely and journalistically feature music across genres,” argues Mark Cooper, the show’s co-creator and founding producer. “If you had read about a great new R&B act, or a great new world music act, or a new guitar band, hopefully you would see them on Later.

“I’ve said before I think it’s a show that’s more BBC than the BBC. It is very steadfast to those public service values.”

Cooper, who worked on Later from 1992 until 2018, has just written a very readable book, Later … With Jools Holland, which charts, as its subtitle spells out, “30 years of Music, Magic and Mayhem”.

Actually, that would be mayhem with a small m. Because while there have been drop-outs, a few fall-outs and the odd reluctant participant (step forward The Jesus and Mary Chain), the story Cooper tells in print and in person is more collegiate than confrontational. Turns out in 30 years tension on set between musicians was not a common occurrence.

“No, very, very rare,” he says when I bring it up. “The opposite. People were really encouraging. Rooting for the show, being inspired by other performances in very different styles.”

Tunstall has a simple explanation for that. “I think that’s also down to Jools. I think the fact that he is such a brilliant musician is the genius in the mix. He’s a brilliant presenter, but he’s a dedicated, passionate musician first.

“And he’s also just got one of the most delightful personalities of any human I’ve ever met. He can just disarm anyone. He’s talking to you as a musician, not as a TV presenter.

“You’re immediately on your back foot a little bit when it comes to doing television shows because you’re not always in control of how you are presented and how you’re made to look in the editing process.

“And I think with Jools and with the programme it was just such a deeply supportive environment for musicians.”

Cooper is also quick to sing Holland’s praises. “He’s very generous, he’s curious, he’s his own man musically. He’s got a very long tail in America going back to touring. Many people grew up on Squeeze in the 1980s, so he’s got a lot of skin in the game himself.

“And he’s the icon of the show’s values. Jools embodies and represents them wonderfully well, I think.

“At the same time that can be slightly divisive because Jools is such a particular kind of person. For some people he’s Marmite. They don’t like the way he interviews people, they don’t like the way he frequently plays with people on the show … Perhaps not quite as frequently as the critics might make out.

“Jools has remained - and I mean this in the most positive way - amateur as a presenter, which as a producer could drive me mad. But I think it was also why the show has kept a kind of innocence. He doesn’t want to smarm up to TV. He rather likes it when things go wrong. He’s excitable in a good way, like a fan is.”

The story of Later is the story of a programme which in the beginning was very much of its time but also ahead of it. Of its time in the sense that it plugged into that post-Live Aid, post-Q magazine notion that the big names of previous generations should be applauded rather than shot down. At the same time, though, its eclecticism looked forward to the 21st century and a post-digital musical landscape where all musical genres are equally available.

The template “was to be curious rather than cool,” Cooper writes in Later … With Jools Holland. “We had lots of cool acts on,” Cooper adds when I bring this up, “but cool is a wonderfully evanescent thing that comes and goes. If we tried just to be cool the show wouldn't have lasted, whereas curious is something else.

“I also think we never missed the boat. Famously, the Whistle Test struggled to come to grips with punk and I think having Stormzy on and embracing Grime in the mid-2015s … There are moments I’m proud I didn’t miss.”

Any programme that has been running 30 years will have its share of critics. But sometimes you wonder if Later gets shot at because there is nothing else to shoot at. Apart from the odd slot on chat shows where do you see music on television?

“I’d like to see more music shows,” Cooper admits. “I think Later has done a job of being every music show in a way by being so broad in its taste. It didn’t set out to do that, but it’s done that. But I just wish there was more music on television, not at the expense of Later, to keep music alive and kicking on television and finding new audiences. But I don’t see broadcasters doing it.”

As it stands, Later is closing in on Top of the Pops (which ran for 44 years). Will it make it that far? Will Holland still be around to present it? (And could it survive without him?)

Cooper suggests another question. “I think the more difficult question is, should it? Are music shows meant to run 30 years? How has the culture of music moved on? And how do you bring in younger generations into music with a show and a presenter that is so established?”

He admits he is too invested in the show to answer that question. And maybe this is not the time to ask it. Maybe it’s time to celebrate the legacy of Later first.

Tunstall is more than happy to do so. Thinking back to her first appearance, she says, “I’ve not done an interview ever where it hasn’t been mentioned. It just seems like the entire planet was watching that episode that night.”

As a result, she’s not playing Fibbers these days. And not just because it’s closed down. She is the first to acknowledge she has the makers of Later to thank for that.

“I am so grateful to them. They totally changed my life.”

Jools’s 30th birthday Bash is on BBC2 tonight at 9pm. Later … With Jools Holland by Mark Cooper is published by William Collins, £25. KT Tunstall’s latest album NUT is out now