Mancur Olson's "Logic of Collective Action" predicts that voluntary action for the common good will only happen in small groups. His theory of the structure and behaviour of organisations fails to account for the UK Labour Party's promotion of, rejection and ultimate compliance with its Political Parties Act (PPERA). I propose a revised computational theory to explain this behaviour. It negates key tenets of Olson's work: that consumption of a common good by one member inevitably reduces the quantity available to another and that negotiation between members does not in general affect a decision to work for the common good. The approach has application in private and public sector service design.