THE thing is, Kathryn Williams says, you don’t know how far you’ve walked until you turn around. For Williams, folk singer, songwriter, mother, Scouser transplanted to the north-east of England, and owner of a voice purer than mineral water, that turns out to be far.

Williams can look back over almost 20 years of making music, in fact. Two decades and more than 14 albums, most, but not all, under her own name.

“I’ve never been nostalgic. I’ve always been searching and moving forward,” she admits. But a 20th anniversary is a notable one and it deserves to be marked. And so, she has, with a sumptuous box set, called Anthology, via One Little Indian.

“It’s been a couple of years in the making. It’s just really lovely to have this opportunity,” Williams tells me as she stands on a train home to Newcastle after an evening in Edinburgh performing at the book festival. “I’m stood in a vestibule of a train pressing my phone really hard against my ear like a complete knob talking about myself to you.”

It’s the morning after the night before. “My brain’s dead,” she says at one point when a Joni Mitchell line she wanted to quote from the album Blue escapes her. “I was drinking until four, so I am feeling a bit rough.”

Anthology is a monster of a thing. But it’s a beautiful monster. The box set has been made possible by the fact that the records she made for Warner Records under a licensing deal have now reverted back to her. Williams’s current label One Little Indian has done her proud. Garlanded by Williams’s own art, it’s an instant (if 20 CDs’ worth of material can ever be described as instant) resume of her mercurial, Mercury-approved career.

It travels from the feather-and-squeak guitar sounds of her minimalist debut Dog Leap Stairs (made for a reputed £80; she thinks it probably cost a bit more than that), via the “Mercury Prize one,” Little Black Numbers (she lost out to Badly Drawn Boy’s The Hour of the Bewilderbeast, but then so did Coldplay and Richard Ashcroft), her covers album Relations where she took on Nirvana and Ivor Cutler, the intimacy of Leave to Remain (possibly my favourite), through to the more recent Crown Electric and Hypoxia, an album inspired by the poet Sylvia Plath and one that even sees the clarity of her voice embedded pleasingly in a sound bed of dust and grit and beats on one track (enough to wish Williams had made a trip-hop album somewhere along the way.)

“It just felt like a labour of love really, “Williams suggests. A way to say thanks to those who have supported her. And, to that end, with each of the official CDs there’s an accompanying bonus disc of live recordings and demos to ensure value for money. “I didn’t want to do a box set which was basically repackaging things that they had, so the reason for the 10 bonus CDs for each CD is for all those people who have those albums and they then can get 10 albums’ worth of stuff they’ve never had and can’t get anywhere else. I didn’t want to rip off my fans really.

“They’re not perfect in any way,” she says of the bonus CDs. “It’s like standing in front of everyone without make-up and saying: ‘Look, this is where little seeds come from.’”

And there are a lot of seeds. Anthology is proof of Williams’s Stakhanovite levels of creativity. She was putting it together at the same time as she was working on a new album of songs to accompany Laura Barnett’s 2018 novel Greatest Hits.

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And that’s only the start of it. “I’ve been making two new albums, a really strange electronic synth record and then a really, really minimal folky record,” she says, beginning to lay out her current output.

“And I’m also writing a set of new canon of Christmas carols with Carol Ann Duffy. And I’ve got a record I’m making with Michelle Stoddart of the Magic Numbers, and then doing a project about Angela Carter for the British Library.

“So, I do all of that and then I write with other artists. I’ve just written an album for a Swedish pop artist called Peter Joback. He’s very theatrical and wonderful. I did an album with a Norwegian band as well, so I like writing for other people.”

What’s the appeal of these left-handed activities, I ask? “When I’m working for another artist it’s often way out of my genre. It’s like tailoring a suit; working out what would be perfect for their voice.”

Her own is remarkable in its own way, of course, unadorned and crystal clear. It’s been in evidence from the very first recordings. It’s curious, she says, listening again to her earliest efforts. “Those albums captured a time. I think I’m a better songwriter now in some ways, but there’s a beauty in the naivety. I had never heard myself being recorded before. I had never been in a studio. I didn’t know what I was doing, and I like that that exists.”

What makes Williams’s story all the more remarkable is the fact that when she started, she struggled with panic attacks and stage fright. It even blighted her Mercury Prize moment in 2000.

“Well, I wish I had been more ready for it in a way,” she says when I bring up the nomination for Little Black Numbers. “I kind of pissed on my chips really, because I couldn’t handle it. People would be booking me to go on loads of TV and radio. And they would phone and say: ‘Are you at the studios?’ And I’d say: ‘No, I’ve had a panic attack. I’m lying on Peterborough station and I’m just going to get on the train and go back home.’

“I used to black out with nerves on stage and I was constantly feeling like an imposter. Imposter syndrome.”

There must have been a real resilience in her then to overcome all of that and keep going to such productive ends.

“It’s so weird. I’ve thought about that a lot. I was just trying to work out why I carried on when it was so painful. I think a lot of people who do this are performers. I think I’m just a writer who represents her songs. I’m not someone who loves the spotlight, likes jumping around and dressing up and all of that. I would like to be able to sing my songs in complete blackness.”

She still gets nervous, she says. “But it’s a different thing now. As you get older you think less about yourself in that way. I feel like what I’m doing is just presenting the songs and just wishing them the best.”

Becoming a parent might be part of that, I suggest. “You realise that early on when you’re pregnant on stage,” she admits. “How am I going to get this baby out of my body?”

Williams’s appeal is based on the unadorned purity of her voice and the quiet intimacy of her songs. In short, she’s the last person you might expect to have been managed by Alan McGee, of Creation Records fame, for a time.

“He was really great. When I was nervous, he’d go: ‘Hey Kath, you know what? Just f*** it.’ And that was great advice. He said he’d manage me forever and I loved him. We didn’t fall out, but it’s just strange when you’re being managed by someone more famous than you, because everyone wants to talk to your manager.”

McGee would even invite her along to gigs and she’d find herself incongruously backstage with the likes of the Libertines at the height of their excess. What, you wonder, did she talk to them about? “I was complaining to them that I would have to leave the party early because I’d been gardening all day and I was a bit tired and I had forgotten to feed my cats.”

Kathryn Williams, it seems, has always gone her own way. It’s only now that we can see how far that’s taken her.

Anthology, by Kathryn Williams is out now on the One Little Indian label.