Mamma Mia!
Scarlett was part of the original cast of the London production of 'Mamma Mia' which opened at the Prince Edward theatre on April 6th , 1999 (previews from March 23rd, 1999) being a member of the ensemble. She stayed with the show for a year to move on then to 'The Witches of Eastwick'.
More information about the musical on Wikipedia.