Detachable locking device for a control valve and method

A method of preventing fluid flow down hole comprising the steps of:

• inserting a body member down hole the body member having a channel through which fluid can flow, a moveable member mounted within the body member, the moveable member comprising an obturating member for selectively obturating the channel, and the moveable member being locked by a detachable locking device in a first position in which the channel is not obturated and fluid flow through the channel is permitted;
• actuating a displacement system to unlock the detachable locking device, wherein said actuation is achieved by landing a ball on a drop ball seat provided in the displacement system, the drop ball seat being coupled to the moveable member and increasing the force of fluid pressure up stream of the drop ball seat so as to unlock the locking device;
• and
• increasing the fluid flow to create a pressure drop over the moveable member and move the moveable member from the first position to a second position in which the channel is obturated and fluid flow through the channel is substantially prevented.