A hydraulic lock for locking the position of an actuation member of a downhole tool relative to the body of the downhole tool is described. The hydraulic lock comprises first and second fluid chambers fluid communication between which is provided by a first fluid conduit. Located with the first fluid conduit is a first logic controlled valve that is moveable between an open configuration and a closed configuration to selectively permit fluid communication between the first and second chambers. The hydraulic lock further comprises a secondary control mechanism that provides a second means to selectively permit fluid communication between the first and second chambers. Incorporating the secondary control mechanism within the hydraulic lock provides a means for increasing the reliability and working lifetime of the hydraulic lock. It also provides a means for reducing the risk of unintentional or false activation of the hydraulic lock.
