A non-return valve assembly (100) for a high-pressure fuel injection system is disclosed. The valve (100) comprises a valve chamber (102) defined in part by a first body (106) and in part by a second body (108) and defining a valve chamber wall (118), an inlet passage (110) formed in the first body (106) and opening into the valve chamber (102) at a valve seat (116) defined by the first body (106), an outlet passage (112), and a valve ball (114) received within the valve chamber (102) and engageable with the valve seat (116) so as to interrupt fluid flow from the outlet passage (112) to the inlet passage (110) through the valve chamber (102). The valve chamber wall (118) comprises a plurality of guide portions (122a) to guide the valve ball (114) in substantially linear movement within the valve chamber (102).