A downhole valve comprises a body defining a through bore and a valve member for location in the body and movable between an open position and a closed position. The valve has a first configuration in which the valve member is normally closed, and will hold a pressure differential in a first direction, and a second configuration in which the valve member is closed and will maintain a pressure differential in the first direction and also in an opposite second direction and a third configuration in which the valve member is locked in the open position until it is deliberately closed.