A pumping mechanism for a fuel pump for use in an internal combustion engine, the pumping mechanism comprising: an inlet valve body comprising a pumping element location bore in which a pumping element is moveable in a longitudinal axis; a cam box comprising a guide bore in which a roller/shoe guide is moveable; wherein the roller/shoe guide is co-operable with a driving mechanism via a roller; wherein the pumping element abuts the roller/shoe guide, and in a pumping stroke, the driving mechanism causes movement of the roller/shoe guide and pumping element, and wherein movement of the pumping element within the pumping element location bore causes fuel in within a pumping chamber provided in the inlet valve body to pressurise; wherein the pumping mechanism further comprises a return spring (114, 214, 314, 414), located around part of the pumping element and within a spring chamber; and wherein a first end of the return spring (114, 214, 314, 414) abuts a first spring seat proximate to the inlet valve body, and a second end of the return spring (114, 214, 314, 414) abuts a second spring seat proximate to the roller/shoe guide, such that the return spring (114, 214, 314,414) urges the roller into contact with the driving mechanism; characterized in that: the first and second ends (148, 150) of the return spring (114, 214, 314, 414) each define an external diameter, wherein the external diameter of the first end of the return spring (114, 214, 314, 414) is greater than the external diameter of the second end of the return spring (114, 214, 314, 414) and, wherein the roller/shoe guide comprises a recess (130, 132) comprising a first section, and a second section which is closer to the driving mechanism than the first section and which is of a smaller diameter than the first section, and wherein the second spring seat is located within the second section of the recess (130, 132). an inlet valve body comprising a pumping element location bore in which a pumping element is moveable in a longitudinal axis; a cam box comprising a guide bore in which a roller/shoe guide is moveable; wherein the roller/shoe guide is co-operable with a driving mechanism via a roller; wherein the pumping element abuts the roller/shoe guide, and in a pumping stroke, the driving mechanism causes movement of the roller/shoe guide and pumping element, and wherein movement of the pumping element within the pumping element location bore causes fuel in within a pumping chamber provided in the inlet valve body to pressurise; wherein the pumping mechanism further comprises a return spring (114, 214, 314, 414), located around part of the pumping element and within a spring chamber; and wherein a first end of the return spring (114, 214, 314, 414) abuts a first spring seat proximate to the inlet valve body, and a second end of the return spring (114, 214, 314, 414) abuts a second spring seat proximate to the roller/shoe guide, such that the return spring (114, 214, 314,414) urges the roller into contact with the driving mechanism; characterized in that: the first and second ends (148, 150) of the return spring (114, 214, 314, 414) each define an external diameter, wherein the external diameter of the first end of the return spring (114, 214, 314, 414) is greater than the external diameter of the second end of the return spring (114, 214, 314, 414) and, wherein the roller/shoe guide comprises a recess (130, 132) comprising a first section, and a second section which is closer to the driving mechanism than the first section and which is of a smaller diameter than the first section, and wherein the second spring seat is located within the second section of the recess (130, 132).