Friction with her bandmates has previously been blamed for her deciding to break away, but she now says there was a lot of pressure on them from within the music industry.
She told The Evening Standard Magazine: ''Looking back, there was a lot of divide and conquer going on. The adult world around us wanted to manage the environment and make it a business, and maybe forgot we were young and our happiness was important.
''We were teenagers. We weren't even old enough to drink! They saw us as interchangeable - it was all about the band, not the individuals, and that's not good for your self-esteem.''
Over the following years all three original members of the band were replaced, but have since reunited as Mutya Keisha Siobhan [MKS], and are preparing to release their debut single 'Flatline'.
Siobhan, 29, explained how her first meeting with Mutya, 28, and Keisha, 28, was a world away from when they had last seen each other.
She added: ''We met up in [upmarket hotel] The Lanesborough. The last time we'd seen each other we'd been teenagers in our tracksuit bottoms having KFC and our parents were there, and here we were having this fabulous dinner and cocktails. We were quite shocked to find we got on really well.''
MKS debut single 'Flatline' is released on September 15th, a full interview with the band in available in this week's ES Magazine, available free today with the London Evening Standard newspaper or at standard.co.uk/lifestyle/esmagazine
Take a look back at October's inaugural event.
The film is expected to continue without Mendes' involvement.