Comparing guidance documents issued by the Office for Fair Access (OFFA), the Higher Education Funding Council for England (HEFCE) and the Office for Students (OfS) over the course of 15 years, I argue that the introduction of a new higher education regulator in 2018 caused a shift in the positioning of widening participation evaluation in HE policy. I suggest that the resulting changes have significant implications for the configuration of key evaluation stakeholder groups and that these reconfigurations, in turn, have implications for the epistemic relationships at play in the evaluation process. In particular, the way in which a mode of evaluation is framed by policy can determine who has the power to shape dominant definitions of meaningful evidence and whose situated forms of knowledge are considered to constitute robust evidence. The ongoing tension between positivist and post-positivist approaches can be eased, I argue, by focusing on the role of delivery practitioners as producers of evidence about 'what works' in their own contexts. I conclude by drawing on other practice-based disciplines such as social work and nursing to suggest that we might learn from work that is already advanced in these areas, which appears to have found a balance between evidence-based practice and practice-based evidence.