A common rail fuel system for an internal combustion engine includes a common rail fuel volume for supplying pressurised fuel to a plurality of fuel injectors of an engine and a unit pump assembly (8) for supplying pressurised fuel directly to the rail volume. The pump assembly has a pumping plunger (10) that is reciprocable within a plunger bore (14) provided in a unit pump housing (16) under the influence of a drive arrangement (18, 20) to cause fuel pressurisation within a pump chamber (12). The drive arrangement includes a drive member (20) coupled to the plunger (10) to impart drive thereto, in use, so that the plunger (10) performs a pumping cycle including a pumping stroke and a return stroke, and a spill valve (46) which is operable to control whether fuel is pressurised within the pump chamber (12) during the pumping stroke of the plunger (10). An outlet valve (58) controls the supply of pressurised fuel from the pumping chamber (12) to the common rail fuel volume in circumstances in which the spill valve is closed.
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