A pump head for a fuel pump assembly is described. The pump head (120,120a) comprises a tappet (134) and a pump head main body (122). The tappet (134) has an internal volume and a tappet base (176). The tappet base (176) is adapted to be co-operable with an engine driven cam so as to, in use, impart drive to the tappet (134). The pump head main body (122) comprises an inlet means for admitting fuel at relatively low pressure and an outlet means for expelling pressurised fuel at relatively high pressure. The tappet (134) is adapted to reciprocate with respect to the pump head main body (122). The internal volume of the tappet (134) forms at least a part of a pump chamber (130) for pressurising fuel to be expelled through the outlet means.