She was hired as a session singer to provide the vocals on the track, so didn't expect credit on the song.
But she hoped the track's success would give her career the boost she wanted.
It didn't. Neither did a second No.1 that year or a Top 10 single in 2017, More Than Friends with James Hype, which did credit her.
James tells Newsbeat he wants to see more "transparency" in the dance industry.
"I think transparency is everything, to be honest. It would be great for the fans to know exactly who is behind the piece of music," he says.
"A lot of dance records rely on authentic, soulful-sounding voices and there are a lot of records where that is almost the biggest part, in dance music."
James says it was a "no-brainer" to share the song credit.
"Whether or not it was going to work, all came down to how good the vocal was on the track," James says.
"As far as I'm concerned, if it wasn't for Kelli-Leigh, that track probably wouldn't exist, or if it did, then it may not have been as successful as it was."
'Are you playing the race card?'
Earlier this week, Kelli-Leigh tweeted about her frustrations, responding to a tweet about black women in dance music.
After nearly 10 years in music she now manages herself and releases solo music on her own record label, Music Core Ltd.
"Funk Butcher's tweet hit home. It struck a chord," Kelli-Leigh says.
"Sometimes it's hard to articulate yourself in the right way without thinking someone's going to go - 'Are you playing the race card?' or 'Are you playing the woman card?'
She says this is stigma all black women carry.
"We've had so many issues of subtle racism here and there that we've lived through our whole lives. A lot of us have adapted to it and ignored it," Kelli-Leigh adds.
"You get to a point where some things just don't seem like a coincidence any more. They can't be. You're not acknowledged."
Kelli-Leigh's words were praised by fans and musicians, including British producer MNEK and other artists who have appeared without full credit on major hits.
"This is the sad truth. Things need to change," another singer-songwriter Carla Monroe commented on Kelli-Leigh's Instagram post.
But this is nothing new.
In 1989 an Italian dance act called Black Box used a vocal sample of soul singer Loleatta Holloway on their worldwide hit, Ride On Time, without permission.
When copyright owners took legal action against the group, they got British singer Heather Small to re-sing the part.
Neither black woman was credited.
In 1991, a model appeared in the video to C+C Music Factory's hit Gonna Make You Sweat, lip-synching to vocals by black singer Martha Wash.
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