Gender balance in computing education is a decades-old issue that has been the focus of much previous research. In K-12, the introduction of mandatory computing education goes some way to giving all learners the opportunity to engage with computing throughout school, but a gender imbalance still persists when computer science becomes an elective subject. The study described in this paper investigates whether introducing pair programming would make a difference to primary-aged girls' attitudes to computing and intent to study the subject in the future. A randomised controlled trial (RCT) was designed and implemented around a 12-week intervention with 785 female pupils between the ages of 8 and 10 years, alongside a qualitative evaluation investigating teachers' and pupils' experience of the interventions and the development of materials and teacher preparation resources. The results of the RCT showed no statistically significant changes in student attitudes towards computing or intent to study further, although the qualitative data indicated that both teachers and pupils found the interventions engaging and enjoyable. Themes emerging from the qualitative data point to the importance of collaboration in supporting a development in pupil confidence. Overall, these results emphasise the societal and systemic barriers around computer science and technology engagement across genders that persist despite many initiatives being implemented over many years.