The provision of all-female computer science outreach programmes is a common strategy used to foster greater interest in the subject for high school aged girls. Based on key factors that affect girls' interest in computer science (CS), outreach programmes often share much in their approach. Nonetheless, there is criticism from a research perspective concerning how programmes evaluate their efficacy, and how the role of pedagogy is under-explored. This article describes the design of CodePlus, a non-formal CS outreach programme based in an Irish University, Trinity College Dublin, and the methods by which the programme is evaluated. This article aims to contribute to this area by reporting on a social constructivist pedagogical model for all-female CS outreach activities, evaluated with a structured research approach. The results from the large sample size (n=856) are positive, with participants showing statistically significant changes in key attitudinal and intentional variables concerning girls' interest in studying computer science.