A valve comprising a first pan (10′) defining a first array of apertures (20′) and a second part (50) defining a second array of apertures (60′), the first pan (10′) being moveable relative to the second part (50) between a first configuration in which passage of a fluid through the valve is substantially prevented and a second configuration in which passage of fluid is allowed. In one embodiment, the first pan (10′) comprises a flexible plate-like member configured to engage a sealing face of the second part (50) when in the second configuration and lock in the second configuration in response to a pressure differential across the valve. The plate-like member is sufficiently flexible to conform to a profile of the sealing face in response to a pressure differential across the valve.