Teacher collaboration, and teacher professional development within this context, has become an area of interest in recent years. In particular, teacher education has seen the rise of collaboration as an effective school-based professional development activity, where in-service teachers plan, observe and reflect on lessons together. The most common form of teacher collaboration is between teachers within one school or department; however, in England networks do exist that involve teachers from different schools (cross-school). Of the small number of collaborations that do extend beyond a single school, few involve teachers from different age phases (cross-phase), and only a very small number of these involve teachers planning lessons together, observing and jointly reflecting on teaching practice. This article analyses and discusses the experiences of two primary school teachers in a cross-school and cross-phase collaborative year-long innovative initiative to speculate about the implications of this type of professional development. Specifically, the article examines whether cross-phase and cross-school collaboration affords primary school mathematics teachers opportunities to develop and addresses what hinders their participation. Implications for future research on primary school teacher collaborative professional development are discussed.