A valve (50) comprising a first part (401) defining a first array of apertures and a second part (2) defining a second array of apertures, the first part (401) being moveable laterally relative to the second part (2) between a closed configuration in which the first and second arrays of apertures are not registered to substantially prevent passage of a fluid through the valve and an open configuration in which the first and second arrays of apertures are registered to allow passage of fluid, wherein the first and second parts (401, 2) are configured to lock in the closed configuration in response to a pressure differential across the valve (50). In one embodiment, the first part (401) is configured to be sealed against the second part (2) by a pressure differential across the valve when the first and second parts (401,2) are locked in the closed configuration.