oriented bolts is a lock 108 which is tightly fixed at each of its ends to the bolts via attachment members that are seen clearly in fig3 and that will be described below as a consequence of this tight fixing the substantially axial displacement of the bolt from the locked to the unlocked position is accompanied by a contraction of lock 108 and the opposite displacement of the bolts is accompanied by an extension of the lock the lock 108 is linked to a control mechanism shown schematically as box 110 the control mechanism may be a separate unit linked to the lock by wire or wireless link and may also by some examples be integral with the lock the control mechanism is adapted to receive an access control signal which may be a knocks based access control signal of the kind disclosed in pct publication no wo 98 39539 in which case the control mechanism may include an acoustic pick up sensor of the kind disclosed in pct publication no wo 01 59238 it should further be noted that the invention is not limited to a control mechanism housing an acoustic sensor picking up a code encoded as a series of knocks and may be designed for picking up codes transmitted through other means such as visible or infrared light in which case the sensor should have a line of sight to the exterior bluetooth radio waves and other communication links the control mechanism by some examples may also be linked to an external socket accessible to an operator and the code may then be.