Abstract: A pump assembly for use in delivering fuel to a common rail of an internal combustion engine, including a pumping plunger (28; 128) which is co-operable with an intermediate drive member in the form of a tappet (34). The tappet (34) is co-operable with an engine-drive cam (11), in use, so as to impart drive to the plunger (28; 128). A cam rider member (38) of generally tubular form has an inner surface which co-operates with the cam (11) and an outer surface which cooperates with the bucket tappet (34). A main pump housing (10) is provided with a radially extending opening (18a) within which is received, at a first end thereof, a separate pump head (20a) and, at a second end thereof, the tappet (34) so that the opening (18a) serves to guide axial movement of the tappet (34), in use. The separate pump head (20a) is provided with a plunger bore (26) which serves to guide movement of the plunger (28; 128) and within which is defined a pump chamber (30) in which fuel is pressurised as the plunger (28) is driven.