Energy markets policy in Great Britain has been largely delegated from elected representatives to a market regulator: the Office of Gas and Electricity Markets (Ofgem). Regulatory legitimacy requires due process and appropriate expertise to expose the regulator to democratic influence. As the legitimacy of regulatory participation processes start to be discussed more intensively in the European context, this timely article examines the relationship between the use of policy formulation tools and the resulting legitimacy gained by an independent market regulator. It employs a detailed case study analysing how participatory policy formulation tools - deliberative focus groups with members of the public, and stakeholder consultations - were used in energy markets policy formulation in Ofgem between 2007 and 2016. Through assessing the actors, venues, capacities and effects associated with selection and use of the tools, it finds there were inequalities of influence between different policy actors which posed a significant challenge to legitimacy.