Since 2014, SEERIH (Science and Engineering Education Research and Innovation Hub) has developed a professional learning programme for primary science subject leaders in Greater Manchester. This responds to the ongoing need for teachers to develop skills and confidence in the teaching, learning and assessment of primary science (Wellcome Trust, 2017) and the standards for teachers' professional development (DFE, 2016). A key aspect of the SEERIH programmes is the range of Science Subject Leader Networks, enabling any leader in a Greater Manchester school to access termly continuing professional development (CPD) meetings. Run in coordination with Local Authority and Teaching School Alliance partners, these are designed and facilitated to inform, inspire and improve the teaching of National Curriculum in England science. This article considers the role of the Network Guardian -- a new approach trialled to gauge the depth of impact of professional learning within SEERIH Primary Science Subject Leader Networks.