A back-flow prevention valve has a sprung valve closure which comes to rest against a valve seat. The sprung valve closure is a ball valve with a locating component for correct positioning on the valve seat. The flat sprung valve closure (36,62) can be repositioned radially at that end furthest from the closure section. The sprung valve closure (38,62) is a spring washer incorporating semi-circular or spiral spring cross-members (44).
