A study of nine headteachers in England, using a research protocol developed and carried out in Ontario, finds implications for the professional development of school leaders with respect to their decision-making to achieve greater equity. The types of professional development that leaders found to be most effective within their particular contexts are identified. The challenges faced by school leaders are explored in relation to their efforts to reduce context-specific achievement gaps. The data generated from semi-structured interviews are examined within the context of the English school system, which is highly neoliberalised and has a performative driven policy landscape. The impact of this context on the practice of school leadership for social justice is discussed and, in particular, how data-based decision-making can be improved through professional development in order to achieve more equitable outcomes, especially for disadvantaged students. The paper has relevance for practitioners in how its findings illuminate professional development practices that school leaders might benefit from and from an argument that reflective and reflexive school leaders are more likely to make objective, critically informed decisions.