A downhole injection device for use in injecting a fluid into a wellbore target location, comprising: a housing defining an inlet for communicating with a source of injection fluid, an outlet for communicating with a target injection location, and a separate reference port for communicating with a reference pressure source; a first valve member mounted within the housing; a second valve member mounted within the housing and defining a flow path to facilitate fluid communication between the inlet and outlet of the housing, wherein the second valve member is movably mounted within the housing between an open position, wherein the first and second valve members (116,118) are disengaged to facilitate fluid communication between the inlet and the outlet of the housing through the flow path, and a closed position, wherein the first and second valve members (116,118) are engaged, thereby preventing fluid communication between the inlet and the outlet of the housing through the flow path; a biasing arrangement that acts upon the second valve member to bias the second valve member toward the closed position so as to decrease flow between the inlet and the outlet; and a sealing arrangement provided between the second valve member and the housing and configured such that fluid pressure at the housing inlet and housing reference port apply a force on the second valve member to cause said second valve member to move relative to the first valve member and vary flow between the inlet and the outlet; wherein, to facilitate movement between the second valve member and the first valve member, the fluid pressure at the housing inlet must exceed the combined force of the housing reference port pressure and the biasing arrangement, such that the position of the second valve member is achieved in accordance with a pressure differential between inlet and reference pressures. a housing defining an inlet for communicating with a source of injection fluid, an outlet for communicating with a target injection location, and a separate reference port for communicating with a reference pressure source; a first valve member mounted within the housing; a second valve member mounted within the housing and defining a flow path to facilitate fluid communication between the inlet and outlet of the housing, wherein the second valve member is movably mounted within the housing between an open position, wherein the first and second valve members (116,118) are disengaged to facilitate fluid communication between the inlet and the outlet of the housing through the flow path, and a closed position, wherein the first and second valve members (116,118) are engaged, thereby preventing fluid communication between the inlet and the outlet of the housing through the flow path; a biasing arrangement that acts upon the second valve member to bias the second valve member toward the closed position so as to decrease flow between the inlet and the outlet; and a sealing arrangement provided between the second valve member and the housing and configured such that fluid pressure at the housing inlet and housing reference port apply a force on the second valve member to cause said second valve member to move relative to the first valve member and vary flow between the inlet and the outlet; wherein, to facilitate movement between the second valve member and the first valve member, the fluid pressure at the housing inlet must exceed the combined force of the housing reference port pressure and the biasing arrangement, such that the position of the second valve member is achieved in accordance with a pressure differential between inlet and reference pressures.