By March 1943 Conant decided that British help would benefit some areas of the project. James Chadwick and one or two other British scientists were important enough that the bomb design team at Los Alamos needed them, despite the risk of revealing weapon design secrets. In August 1943 Churchill and Roosevelt negotiated the Quebec Agreement, which resulted in a resumption of cooperation between scientists working on the same problem. Britain, however, agreed to restrictions on data on the building of large-scale production plants necessary for the bomb. The subsequent Hyde Park Agreement in September 1944 extended this cooperation to the postwar period. The Quebec Agreement established the Combined Policy Committee to coordinate the efforts of the United States, United Kingdom and Canada. Stimson, Bush and Conant served as the American members of the Combined Policy Committee, Field Marshal Sir John Dill and Colonel J. J. Llewellin were the British members, and C. D. Howe was the Canadian member. Llewellin returned to the United Kingdom at the end of 1943 and was replaced on the committee by Sir Ronald Ian Campbell, who in turn was replaced by the British Ambassador to the United States, Lord Halifax, in early 1945. Sir John Dill died in Washington, D.C., in November 1944 and was replaced both as Chief of the British Joint Staff Mission and as a member of the Combined Policy Committee by Field Marshal Sir Henry Maitland Wilson.