LOCKING MECHANISM FIELD OF THE INVENTION This invention relates a locking mechanism particularly for locking a pair of articles that slide with respect to one another, such as sliding doors, windows or panels. 5 BACKGROUND OF THE INVENTION Locks or latches, designed to latch and unlatch a pair of sliding articles such as doors, that typically allow entry and exit from an enclosed area, have existed for many years. Early designs of these types of sliding door locks simply added a tongue or hook that engaged a keeper on an opposite door which was activated by a knob or switch on 10 the door supporting the lock, These types of door locks were relatively easy to disengage and therefore did not provide great security. Other locks incorporated a key cylinder so that the hook or tongue could be latched and unlatched through operation of a key inserted into the key cylinder. More recent versions of sliding door locks included a deadlocking facility so that in is a deadlocked position a snib button, which is nornmally used to latch and unlatch the tongue from the keeper, could not be operated to unlock the door. Only by using the key could the lock be undeadlocked. This provided greater security to the area accessed through the doors incorporating the deadlocks. However some existing locking devices are overly complex and have many parts 20 that are not only expensive to manufacture but can be prone to excessive wear and breakage. Use of many parts tends to create bulk within a cartridge of the locking mechanism, which is inconvenient when repairs are necessary and adds to the overall weight of the mechanism. The present invention seeks to overcome one or more of the above disadvantages by 25 providing an improved locking mechanism, particularly for locking a pair of sliding articles together. The above references to and descriptions of prior proposals or products are not intended to be, and are not to be construed as, statements or admissions of common general knowledge in the art in Australia. 30 SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided a locking mechanism for locking a pair of articles together, the articles adapted for relative sliding movement, the locking mechanism including: 5 a cassette; a snib operator and a snib button linked to said snib operator; an actuator housed within said cassette and adapted for movement between a first position in which the mechanism is unlocked and a second position in which the mechanism is locked, said actuator linked to said snib button by said snib operator; 10 a first hook element and a second hook element connected to said cartridge and engageable by said actuator to rotate said first and second hook elements into and out of said locked and unlocked positions; a drive mechanism adapted for rotational movement upon operation of a key inserted into a key cylinder; 15 coupling means for coupling said drive mechanism to said actuator to enable said locking mechanism to move between the locked position, the unlocked position and a deadlocked position upon rotation of said drive mechanism, said coupling means including a slot or recess; and a stop device disposed for movement in and out of said slot or recess, wherein 20 said deadlocked position is reached when said stop device is engaged with said slot or recess of said coupling means; wherein the coupling means includes first gear means having a plurality of teeth adapted to engage second gear means having a plurality of teeth on said actuator to convert rotary motion into linear motion and move said actuator. 25 According a second aspect of the invention, there is provided a locking mechanism for locking and unlocking a pair of sliding articles, said locking mechanism including: a cassette; a first hook element and a second hook element located in the cassette; 30 an actuator located in said cassette and engageable with each of said first and second hook elements, said actuator moveable within said cassette to move said first and second hook elements between a first unlocked position and a second locked position; a drive mechanism actuated for rotational movement by a key cylinder upon insertion of a key into the key cylinder; 35 coupling means linked to said drive mechanism and said actuator enabling movement of the actuator to attain the unlocked position, the locked position or a 2 deadlocked position upon rotation of said drive mechanism, said coupling means including a slot or recess; and a stop device disposed for movement in and out of said slot or recess, wherein said deadlocked position is reached when said stop device is engaged with said slot or 5 recess of said coupling means; wherein the coupling means includes first gear means having a plurality of teeth adapted to engage second gear means having a plurality of teeth on said actuator to convert rotary motion into linear motion and move said actuator. The coupling means may include first gear means adapted to engage second gear 10 means on said actuator to convert rotary motion into linear motion and move said actuator. Turning said key in a first rotational direction can enable the drive mechanism to rotate the first gear means to move the first and second hook elements to the unlocked position, and wherein further turning said key in an opposed second rotational direction 15 enables the drive mechanism to rotate the first gear means to move the first and second hook elements to the locked position, whereupon further rotation in the second rotational direction enables a stop device to engage said slot or recess in said coupling means to deadlock the locking mechanism such that no movement of the actuator is possible. The coupling means may further include a disc attached to or integrally formed 20 with the first gear means, the disc and first gear means having an aperture therethrough, said slot or recess is formed in a periphery of said disc, and said stop device has a lug that protrudes into the slot or recess to deadlock the locking mechanism. The stop device may reside in a stop housing in said cassette and is biased by a spring to move the lug into the slot in said disc. The stop device may be moved against 25 said spring by a cam located on the drive mechanism in order to release said lug from said slot in the disc. The actuator preferably has a first portion, a second portion and a third portion intermediate the first and second portions, said first hook element engageable with the first portion, said second hook element engageable with the second portion, and said third portion having said second gear means, said second gear means in 30 the form of a rack gear. When the cam engages with said stop device, said locking mechanism can be moved between said unlocked and locked positions. The locking mechanism may further include a member for preventing movement of the actuator between said first and second positions when said pair of articles are 35 separated from one another and allowing movement of the actuator between said first and second positions when said pair of articles abut one another. The member preferably has 3 a button that is biased by spring means to reside outside the cassette when the articles are separated and enable the button to be pushed inwardly into the cassette upon contact by the other article when said other article abuts the article to which the cassette is fitted. The member may have a first arm and a second arm substantially parallel to the first arm, 5 the first arm and the second arm joined by a web, said button extending from said web in an opposite direction to each of the first arm and the second arm. Each of said first arm and said second arm can be supported by respective brackets affixed to an inner face of a cover for the cassette. When the button is extended out of the cassette, a lug on either of said first arm or said second arm can engage a post on the actuator to prevent movement 10 of the actuator. Each of said first and second elongate members may be biased by respective springs to keep the elongate members inside the cassette when the first and second hook elements are contained within the cassette in the unlocked position. The locking mechanism may further include a deadlock indicator mechanism for indicating to a user a deadlocked state of the locking mechanism, said deadlock indicator is mechanism having first and second elongate members that protrude from the cassette to indicate the deadlocked state. Whereupon rotation of each of the first and second hook elements, this may force respectively the first and second elongate portions outwardly of the cassette until the deadlocked state is reached whereby the first and second elongate portions extend fully out of the cassette and are only able to be released from the 20 deadlocked state by a user using a key to turn the drive mechanism. The locking mechanism may further include a cassette housing for housing the cassette, a lock body encasing or attached to the cassette housing, and a handle attached to the lock body. BRIEF DESCRIPTION OF THE DRAWINGS 25 A preferred embodiment of the invention will hereinafter be described, by way of example, with reference to the drawings in which: Figure 1 is a perspective view showing the separate components of a locking mechanism in accordance with an embodiment of the invention; and Figure 2 is an enlarged view of a part of Figure 1 showing the components that fit 30 within a cassette of the locking mechanism of Figure 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figures 1 and 2 there is shown a perspective view of the components of a locking mechanism and door furniture (100) used to lock two sliding articles together, 4 such as doors. The embodiment is described solely in relation to doors, but equally can apply to other articles such as windows and panels. The locking mechanism and door furniture includes a body (102), a handle (104), a cassette housing (106), a cassette (108) and a cassette cover (110). The pals of the lock that are housed within the cassette (108) 5 are shown between the cassette (108) and the cassette cover (110). A snib button (112) fits over a snib frame (114) which is attached to a portion of body (102). A snib operator (116) includes a first arm (118) and a second arm (120) connected by a web (122). Formed in an opposite direction to the aims (118, 120) and depending from the web (122) is a snib bar (124) which fits within and locks into a recess (126) in the snib button 10 (112), The recess (126) is bounded by a pair of hooks (128, 130). Up and down movement of the snib button (112) by an operator will correspond to an up and down movement of the snib operator (116). Each of the arms (118, 120) respectively abut portions of actuator (132). Specifically first arm (118) fits within recess (134) of the actuator (132) and second arm (120) fits within recess (136) of actuator (132). Arms 15 (118, 120) protrude through corresponding slots of the cassette housing (106) and the cassette (108). In this manner the user operating the snib button (112) can move the actuator (132) up and down in order to lock and unlock the locking mechanism as will be described hereinafter, A first hook element (138) has a base portion (140) and a tongue (142). A second 20 hook element (144) has a base portion (146) and a tongue (148). A post (not shown) on the right side of base member (140) (as seen in Figures 1 and 2 ) engages slot (150) and ridge (152) on a first portion (151) of actuator (132) in order to move the hook element (138) from an open position to a closed or locked position whereby the actuator (132) is pushed upwardly so that tongue (142) protrudes through slot (154) of housing (106) and 25 recess (164) of cassette (108). Similarly post (156) on base member (146) of hook element (144) engages in slot (158) and ridge (160) of a second portion (159) of actuator (132) such that upward movement of the actuator (132) forces rotation of the hook element (144) from an open position to a closed or locked position as tongue (148) protrudes through slot (162) of housing (106) and recess (166) at cassette (108). Thus, in 30 a fully opened position both hook elements (138, 144) are completely housed within cassette (108) and therefore within housing (106). In a locked or closed position the tongues (142, 148) respectively protrude through recesses (164, 166) on cassette (108) and respectively through the slots (154, 162). The actuator (132) is mounted within 5 cassette (108) against the inside of front face (168) and thereafter hook element (138) is positioned on post (170) via aperture (172) and hook element (144) is positioned on post (173) through aperture (174). The position of the post (not shown) on hook element (138) and post (156) on hook 5 element (144) is such that the engagement with the actuator (132) when the actuator (132) moves upwardly is such that each of the hook elements (138, 144) move from a locked position to an unlocked position residing within the cassette (108). In this instance hook element (138) turns anti-clockwise (as seen from the right) and hook element (144) turns clockwise, Similarly downward movement of the actuator (132) 10 moves the tongues (142, 148) from an open position to a locked or closed position such that hook element (138) moves in a clockwise direction (as seen from the right) while hook element (144) moves in an anti-clockwise direction. When each of the tongues (142, 148) are in a locked or closed position they are latched to a corresponding keeper which is provided usually in a housing, affixed to 15 another sliding door (or window) adjacent the sliding door (or window) to which the locking mechanism is attached. Shown in Figure 1 is a key cylinder (174) which is operated by a key from the right side or front of the cylinder (174). Located at a rear side or left side of the key cylinder (174) is a projection (176) which engages with a shaft (178) of drive mechanism (180). 20 A coupling means (167) including a first gear (186) and a disc (184) couples the drive mechanism (180) to a second gear (188) on actuator (132), The shaft (178) protrudes through aperture (182) of disc (184) which has attached thereto or is integral with the first gear (186) which effectively is a pinion gear also having an aperture therethrough. The gear (186) interacts and meshes with teeth of second gear (188) located on a third 25 portion (190) of actuator (132) between the first portion (151) and the second portion (159). In effect the gear (188) is a rack gear and together with gear (186) form a rack and pinion gear arrangement. Rotation of the key rotates the drive mechanism (180) and disc (184) which, in turn, translates the circular motion of gear (186) into a linear motion against gear (188) to move the actuator (132) up and down thereby moving the lock 30 mechanism between an open condition, closed condition and a deadlocked condition to be described hereinafter. The drive mechanism (180) is formed by shaft (178) affixed to a rim (185) by spokes (183). The shaft (178) has aperture (181) and located inwardly from the perimeter of the 6 shaft (178) into aperture (181) is a ridge (179) that extends along the whole length of the shaft (178). The ridge (179) together with lug (189) interact with the projection (176) on the key cylinder (174) in order to turn the drive mechanism (180), Lugs (185, 187) on disc (184) fit within corresponding spaces on the drive mechanism (180) between 5 opposed spokes (183). In this manner pressure applied on the lugs (185, 187) by the corresponding spokes (183) turn the disc (184) and correspondingly turn the gear (186), The following movement of the hook elements (138, 144) under the action of turning a key in key cylinder (174) will be described in a directional manner such that clockwise refers to turning the key clockwise as it is inserted into the key cylinder (174) from the 10 right side. An anti-clockwise direction means that the key is turned left when the key is inserted at the right side of key cylinder (174). When each of the hook elements (138, 144) are entirely enclosed in the cassette (108), that is in an open or unlocked position, cam (191) bears against the outer surface of protuberance or lug (198) of stop device (192) which has pushed the protuberance 15 (198) backwards within the confines of its housing (202) in cassette (108) against the force of spring (194) which abuts a back portion (193) of stop (192), The backward movement of protuberance (198) enables movement of the actuator (132) in a downward position, when a key in key cylinder (174) is turned clockwise, and continued rotation of the drive (180) in a clockwise direction forces the hook elements (138, 144) into a locked 20 position just as cam (191) has travelled far enough so that the protuberance (198) can spring back against the inside face of rim (185). Continued clockwise motion of the drive mechanism (180) and the disc (184) aligns the slot (196) with lug (200) which is a deadlocking position. In this position no movement of the actuator (132) can take place and therefore the hook elements (138, 144) remain in a deadlocked position unable to 25 move, The only way that the hook elements can be moved from the deadlocked position is by rotating the key in an anti-clockwise direction such that when the cam (191) again contacts the protuberance (198) it depresses it outwardly or backwardly against the spring (194) so that lug (200) moves out of slot (196) and enables movement of the hook elements (138, 144) inwardly towards the inside of the cassette (108) while the cam (191) 30 is in contact with the protuberance (198), In this motion the actuator (132) is moving upwardly through the rack and pinion gear arrangement (186, 188). In a fully closed position the post (156) on hook element (144) is engaged within the slot (158) and when hook element (144) is in an open condition the post (156) is moved 7 along ridge (160). Similarly, in the fully open position, a post (not shown) on the right side of base (140) of hook element (138) resides in slot (150) and when the hook element (138) is moved to an open or locked position, the post on the right side of base (140) moves along ridge (152). 5 Member (204) is used to prevent the movement of actuator (132) in an up and down motion when member (204) is depressed inwardly upon the action of the doors closing so that button (206) is pressed by the other door against springs (216, 218) that abut at their outer ends against the inside of face (224) of cassette (108). When the doors are pulled apart or the windows are pulled apart, the member (204) springs outwardly against the 10 bias of springs (216, 218) so that the actuator (132) cannot move up or down as a lug (not shown) located on the inside surface of second arm (208) abuts against a lower surface of post (226) located at the lower end of gear (188), that prevents movement of the actuator (132). When the member (204) is pushed inwardly, the lug is released or is disengaged from the post (226) so that the actuator (132) can move up and down. Thus in a closed is position, the actuator (132) can be moved and in open position when the doors are apart the actuator (132) cannot be moved up and down. It is to be noted that a lug may be positioned on the inside surface of first arm (210) or anywhere on the member (204) and engage a post (instead of post 226) located anywhere on the actuator (132). Each of the springs (216, 218) reside partly in recesses (220, 222) located on 20 respective first and second arms (210, 208) of the member (204), the first arm (210) and second arm (208) being substantially parallel and joined by a web (209). The button (206), extends from web (209) in a direction opposite to the first and second arms (210, 208), and protrudes through aperture or recess (212) in cassette (108) and recess (214) in cover (110). First arm (210) has a portion that engages with, and is held in position 25 against the cover (214), by an L-shaped bracket (211) mounted to the interior face of cover (214). Second arm (208) has a portion that engages with, and is held in position against the cover (214), by an L-shaped bracket (213) mounted to the interior face of cover (214), A deadlock indicator mechanism is provided through the use of first elongate 30 member (230) and second elongate member (232). Each of the first elongate member (230) and second elongate member (232) are naturally biased by respective springs (234, 236) to be located within the confines of the cartridge (108). Spring (234) resides in slot (235) and spring (236) resides in slot (237) of the cartridge (108), Thus in the open 8 position when the hook elements (138, 144) are positioned wholly within the interior of the cartridge (108), which is the unlocked position, spring (234) pushes out against post (231) and spring (236) pushes out against post (233) to maintain the first elongate member (230) and second elongate member (232) respectively inside cartridge (108). 5 When drive mechanism (180) together with operation of the gems (186, 188) urge the hook elements (138, 144) to a closed or locked position, lug (239) on hook element (144) presses against surface (240) of the second elongate member (232) and pushes it in an outwardly direction, out of the cartridge (108) such that post (233) compresses spring (236). Once the hook element (144) is in a locked position or a deadlocked position 10 (where the hook element 144 cannot be moved), the lug (239) is still in contact with surface (240) to keep the second elongate member (232) protruding out of the cartridge (108) and casing (106) through aperture (244) in the casing (106), Similarly lug (238) on hook element (138) presses against surface or face (245) of the first elongate member (230) which forces post (231) to compress against spring (234) which moves the first 15 elongate element (230) in an upward direction and remains outside the housing (106) through the aperture (242) in casing (106) while the hook element (138) is in a locked position or a deadlocked position (where the hook element 138 cannot be moved) extending through slot (154). Thus each of the elongate members (230, 232) provide an indication to the user, by 20 remaining outside of the cartridge (108) and housing (106), that the lock is in a deadlocked state. Performing the reverse operation by turning the hook elements (138, 144) inwardly, under control of a key in key cylinder (174), so that they reside in the cartridge (108), causes the respective lugs (239, 238) to disengage from respective surfaces (240, 245) so that under the action of respective springs (236, 234) the second 25 elongate member (232) and the first elongate member (230) are forced back into the interior of the cartridge (108). It is to be noted that the locking mechanism, together with the cassette housing (108), lock body (102), handle (104) can be used on a left door (window) or a right door (window) as it is symmetrical and can be turned upside down to suit left-hand and right 30 hand configurations. The present invention provides a lock that is lighter and provides more working space within the lock cassette compared to prior sliding door or window locks. 9