Released in April 2004 and produced by James Morgan and Juliette Pochin, Katherine's classical chart-topping debut album Première is a mix of old standards including Ave Maria and The Lord Is My Shepherd, plus a smattering of traditional Welsh songs and new interpretations of classic tunes by Handel, Bach, Erik Satie and others.

In 2004 she performed at Westminster Cathedral, Sydney Opera House, Royal Albert Hall and Cardiff's Millennium Stadium. In October the same year, Katherine's second album, the aptly-titled Second Nature, was released, reaching number 16 in the UK pop charts.

2005 saw appearances at Live 8 in Berlin the G8 concert in Murrayfield, Scotland, and an opening appearance at Tsunami Relief Cardiff. The also became the new forces sweetheart, performing at the VE Day concert at Trafalgar Square, at which she was introduced by Dame Vera Lynn.

Katherine's third album, Living A Dream, was released in October 2005.