Pump unit

1. A pump unit for a fuel injection system, the pump unit comprising:

an inlet valve member, an outlet valve, a supply line for supplying fuel, a pumping chamber, and a plunger axially moveable within a barrel for pressurising fuel in the pumping chamber such that the plunger forms a seal with the barrel such that the pumping chamber is defined by the barrel;

the inlet valve member being movable between a first position and a second position within the barrel such that the inlet valve member radially forms a seal with the barrel to seal the pumping chamber, wherein a top surface of the plunger is below the inlet valve member;

wherein the inlet valve member has an aperture formed therein, the aperture providing a first fluid pathway between the pumping chamber and the supply line when the inlet valve member is in said first position, and the aperture providing a second fluid pathway between the pumping chamber and the outlet valve when the inlet valve member is in said second position;

wherein the outlet valve comprises a movable outlet valve member and an outlet valve body; and

the inlet valve member forms a seal with the outlet valve body when the inlet valve member is in said second position

wherein said inlet valve member comprises:

a body portion located sealingly in the barrel; and

a head portion located in a low pressure chamber which receives fuel from the supply line.

2. A pump unit as claimed in claim 1 further comprising a return spring located outside of the pumping chamber for biasing the inlet valve member to the second position.

3. A pump unit as claimed in claim 2 wherein the return spring radially surrounds the head portion of the inlet valve member.

4. A pump unit as claimed in claim 1 wherein the aperture extends axially through the inlet valve member.

5. A pump unit as in claim 1 further comprising a sealing ring movably mounted on the plunger.

6. A pump unit as in claim 5 , wherein the sealing ring is movable axially within a recess provided around the plunger.