UK scientific advice on the possible health risks of mobile phones has embraced (or seems to be embracing) broader engagement with interested non-experts. This paper explains the context of lost credibility that made such a development necessary, and the implications of greater engagement for the construction (and expert control) of "public concern." I narrate how scientific advice matured from an approach based on compliance with guidelines to a style of "public science" in which issues such as trust and democracy were intertwined with scientific risk assessment. This paper develops existing conceptions of the "public understanding of science" with an explanation based around the co-production of scientific and social order. Using a narrative drawn from a series of in-depth interviews with scientists and policymakers, I explain how expert reformulation of the state of scientific uncertainty within a public controversy reveals constructions of "The Public," and the desired extent of their engagement. Constructions of the public changed at the same time as a construction of uncertainty as solely an expert concern was molded into a state of politically workable public uncertainty. This paper demonstrates how publics can be constructed as instruments of credible policymaking, and suggests the potential for public alienation if non-experts feel they have not been fairly represented. © SAGE Publications.