The Co-operative Party, which represents the interests and ideas of the co-operative movement in British politics, has been the sister party of UK Labour since 1927. Largely ignored by scholarship, it has been on occasion the third-largest party grouping in the House of Commons and represents a social movement with formal members numbering in the millions. The unusual Labour/Co-operative relationship was tested during the New Labour period, with the Co-operative Party gradually establishing itself as a trusted sidekick and a source of policy ideas, despite some initial tensions. This article examines two historical instances where the party proved decisive in influencing public policy; the “Thomas Bill” in 2001–2002, and the creation of Co-operative Schools during the 2007–2010 Brown premiership. In each case, the activities of Co-operative Party-linked ‘policy entrepreneurs’ were key in the manufacture and exploitation of ‘windows of opportunity’ for policy change. The paper makes two core conclusions, one empirical: that the Co-operative Party was able to influence New Labour’s public policy direction in keeping with its founding objectives. The second is theoretical: that recent trends in Multiple Streams Analysis are reinforced, and that in smaller policy ‘subsystems’, skilled policy entrepreneurs can play a greater role in the creation of windows of opportunity for policy change than the original theory implies.