A Coupler TECHNICAL FIELD The present invention relates to a coupler. BACKGROUND ART A coupler is a device used to secure a work attachment to a work vehicle. They generally have jaws that receive pins on the work attachment. At least one of the jaws is moved by an actuator. This allows the jaws to engage and release the pins thereby securing and releasing the work attachment to the coupler as required. The actuator applies a driving or engagement force to the moveable jaw to retain the pin therein. Generally another jaw of the coupler faces in the opposite direction to the moveable jaw. Therefore the driving / engagement force of the actuator also forces another pin on the work attachment into another jaw of the coupler. However, if the actuator fails then the moveable jaw can move and release the pin. This is referred to as lack of engagement force and provides a significant health and safety risk. There have been a number of recent high profile accidents involving failures such as this causing injury to people. Lack of engagement force is caused by failure of an actuator. This can be for several reasons including loss of hydraulic pressure through leaks or other damage. Therefore, it is known to have locking systems to secure a moveable jaw. These protect against failure of actuators by securing the moveable jaw with respect to the coupler to retain the pin in the jaw. 1 One example of these devices is that disclosed in PCT Application No. GB/2007/003324 to Miller UK Limited. This coupler has a main body to support a pivotal locking member. The locking member prevents a pivoting jaw from moving should the actuator fail. This is achieved by gravity biasing the locking member downwards so that it abuts the jaw thereby holding this and preventing release of the pin. The locking mechanism of the Miller coupler can be released by moving the coupler through a number of steps. These steps involve inverting the coupler so that gravity causes the member to pivot away from the jaw. This allows the jaw to be retracted by the actuator. However, it is an inherent problem of this type of coupler that this must be inverted to enable the jaw to release the pin. This means that it can be a time consuming and awkward process to release the work attachment from the coupler. In addition, relying on gravity to move the locking member means that the system is not fail safe. For instance, dirt or debris may hinder movement of the locking member and prevent it securing and/or releasing the jaw. Yet a further failing of the available couplers is that they are generally configured to work attachments having a predetermined pin separation. Therefore the couplers are not able to be used with different work attachments where the pin spacing varies. This can be a significant limitation on the available couplers. An additional limitation to the effectiveness of similar devices is that they are designed specifically for use with a fixed coupler. Many modern couplers now incorporate a tilting section which permits the attachment to be angled up to 90 degrees in each direction. Any angle less than perpendicular will reduce the effectiveness of a gravity operated locking member. Therefore, it would be advantageous to have a locking mechanism to secure a jaw 2 with respect to a coupler to ensure that a pin is retained therein. In addition, it would be advantageous to have a coupler which addresses the issues with the prior art. Alternatively it is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. Throughout this specification, the word "comprise", or variations thereof such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only. DISCLOSURE OF THE INVENTION A coupler, including a body, 3 a moveable jaw, an actuator configured to move the moveable jaw through a path of movement so as to engage a pin on a work attachment, a locking mechanism configured to secure the moveable jaw at multiple positions that are spaced apart from each other along the moveable jaws' path of movement, and wherein the multiple positions are such that if the actuator loses engagement force that the moveable jaw does not move sufficiently to completely disengage a pin of a work attachment received in the jaw on loss of engagement force in the actuator, and wherein the coupler is configured so as to ensure that the locking mechanism is in a release position to enable deliberate expansion or contraction of the actuator to move the moveable jaw. According to one aspect of the present invention, there is provided a coupler, including: a body, a jaw to receive a pin of a work attachment and thereby secure the work attachment to the coupler, an actuator to move the jaw with respect to the body, a locking mechanism to secure the jaw with respect to the work attachment, characterised in that the actuator moves the locking mechanism to a release position prior to moving the jaw. 4 According to another aspect of the present invention, there is provided a method of securing a work attachment to a coupler, including the steps of: (a) using an actuator to move a jaw of the coupler so as to engage a pin on the work attachment; (b) using a locking mechanism to secure the jaw with respect to the body; (c) causing the actuator to move the jaw; the method characterised by the step of (d) moving the actuator so as to move the locking mechanism to a release position prior to moving the jaw at step (c). According to another aspect of the present invention, there is provided a coupler, including: a body, a jaw to receive a pin of a work attachment and thereby secure the work attachment to the coupler, an actuator to move the jaw with respect to the body; a locking mechanism to secure the jaw with respect to the work attachment; characterised in that the locking mechanism secures the jaw with respect to the work attachment to prevent movement of the jaw in the case of loss of engagement force in the actuator. In a preferred embodiment the present invention may be incorporated to the improved coupler subject of the applicant's co-pending New Zealand Patent Application No. 572477. However, this should not be seen as limiting and the present invention can be incorporated 5 into other couplers. In particularly preferred embodiments, the present invention may include a second locking mechanism configured to secure a pin of a work attachment in the fixed front jaw. In a particularly preferred embodiment the present invention is used with the "primary jaw" of a coupler and reference will be made herein. The term "primary jaw" is a term of the art generally understood as referring to a moveable jaw of a coupler. This is as should be understood by those skilled in the art. Preferably, the machine may be an excavator or other construction vehicle. Reference herein will be made to the machine as an excavator. However, the present invention can be used with other types of machines where releasable work attachments are utilised, including graders and bulldozers, loaders, tractors, and scrapers. Throughout the present specification, reference to the term "work attachment" should be understood as meaning an implement for performing a task. Work attachments generally include two or more pins engaged by the coupler's jaws. That engagement secures the work attachment to the machine. In a preferred embodiment the work attachment may be a digger bucket as should be known to those skilled in the art. Alternatives for the work attachment include vibration compactors, and grapples used in the forestry industry for grasping and manipulating logs, hole boring augers, clamps, rotating buckets, work platforms, mowers, and hedge cutters. However the foregoing should not be seen as limiting and alternatives are envisaged. 6 These include graders and bulldozers, loaders, tractors, and scrapers. Throughout the present specification reference to the term "coupler" should be understood as meaning an assembly to secure a work attachment to an excavator. This is as should be known to those skilled in the art. In a preferred embodiment the coupler has two jaws facing in opposite directions. However it is also envisaged that the jaws could face in the same direction. The jaws will be discussed in more detail below. In a preferred embodiment the coupler may have a body to hold and/or support the components of the coupler. In a preferred embodiment the body may be moveably mounted to an excavator arm. This may occur using techniques or components as should be known to those skilled in the art including a quick hitch. In a preferred embodiment the body may include a path to allow movement of the jaw with respect to the body. The path may be a channel and/or cavity through which the jaw can move. This aspect should become clearer from the following description. However, the foregoing should not be seen as limiting and alternatives are envisaged. These include embodiments where the body does not include a path where the jaw is external to the body. Throughout the present specification reference to the term "jaw" should be understood as meaning a component to engage the pin of a work attachment. This is as should be known to those skilled in the art. In a preferred embodiment one of the jaws is moveable with respect to the body while one of the jaws is formed in the body. 7 In a particularly preferred embodiment, the moveable jaw may be formed in, or attached to, a slide. In this embodiment the slide moves within the path in the body. However alternatives are envisaged including a pivoting jaw, or a jaw external to the body. Throughout the present specification reference to the term "actuator" should be understood as meaning a component that can move the jaw with respect to the body. In a preferred embodiment the actuator may be a hydraulic cylinder as should be known to those skilled in the art. However, the actuator may also be a pneumatic cylinder, a helical actuator, a threaded manual actuator, or chain drive assemblies. Therefore, the foregoing should not be seen as limiting. In a particularly preferred embodiment the hydraulic cylinder may be connected to the locking mechanism such that deliberate movement of the actuator moves the locking mechanism to a release position. This allows the actuator to move the jaw with respect to the body. This should become clearer from the following description. Preferably, deliberate expansion or contraction of the actuator moves the moveable jaw between a pin engaging position and a pin releasing position, and wherein the multiple positions are positions for the moveable jaw that are between the pin engaging position and the pin releasing position. Throughout the present specification reference to the term "locking mechanism" should be understood as referring to a component to secure the jaw with respect to the body. In a preferred embodiment, the locking mechanism may help to ensure that a pin is sufficiently held within the moveable jaw so that the work attachment does not disengage 8 from the coupler in the case of loss of engagement force in the actuator. However in normal operation as the actuator moves the jaw, it moves the locking mechanism to the release position thereby allowing the jaw to move so as to release the pin. In a particularly preferred embodiment the actuator is connected or linked to, the locking mechanism. That connection or link helps to ensure that the locking mechanism does not move to a release position until there is deliberate movement of the actuator. The term "deliberate movement" refers to movement intended by the excavator operator. Preferably the connection of the locking mechanism and actuator is such that the actuator can move slightly without moving the locking mechanism to the release position. This ensures that if the actuator contracts (or expands) due to loss of engagement force that it will not move the locking mechanism to the release position. However, in the preferred embodiment deliberate movement of the actuator can still move the locking mechanism to the release position thereby allowing the jaw to move. This feature is useful in protecting against loss of engagement force which would otherwise result in the jaw releasing the pin causing the work attachment to disengage. In a preferred embodiment the locking mechanism is formed from member(s) and/or pawls which engage with recess(es). In a particularly preferred embodiment, the member(s) and/or pawls are pivotally mounted to the jaw or body. The member(s) can therefore extend into the recess(es) on the body or jaw, thereby securing the jaw with respect to the body. In a particularly preferred embodiment the locking member(s) and/or pawls are biased into a locking position. This may be achieved using biasing elements such as springs or compressible material detents. These components apply an urging force to the locking 9 member(s) forcing these towards the recesses. Therefore once the pawls and/or locking members align with the recess they engage. However this should not be seen as limiting as alternatives are envisaged. In a particularly preferred embodiment the locking mechanism may be multi-centred. The term "multi-centred" should be understood as meaning that the locking mechanism can function with variations in pin spacing on work attachments. For instance, the locking mechanism can secure the jaw at different positions along the length of the path. In a preferred embodiment this may be achieved by having multiple recess(s) along the length of the path. The member(s) and/or pawls engage the recess(es) to secure the jaw. This is advantageous as it allows the locking mechanism to operate with different work attachments which may have pins positioned at different spacings. However the foregoing should not be seen as limiting and alternatives are envisaged. Those include a different mechanism for providing a multi-centred locking mechanism, or couplers that do not have multi-centred locking mechanisms. BRIEF DESCRIPTION OF DRAWINGS Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which: Figure 1 an end perspective view of a slide according to the present invention Figure 2A is a side perspective view of a coupler according to the present invention 10 having a locking mechanism in a release position; Fiqure2B is a side view of a coupler according to the present invention having a locking mechanism in the locking position; Figure 3 is an exploded view showing components of the present invention; Figure 4 is a side view of a locking member; Figure 5A-D show a side cross sectional view of an alternate embodiment in operation. BEST MODES FOR CARRYING OUT THE INVENTION The present invention provides an improved coupler (1). The aspects of the coupler (1) will be described by reference to its components in the order in which they are assembled. A body (2) houses the components of the coupler (1). The body (2) has side walls (3) and end walls (not shown). The walls define a cavity (5) to receive a slide (6). Flanges (7, 8) on the body (2) have apertures (9, 10) forming part of a quick hitch (not shown). The quick hitch facilitates securing the coupler (1) to an excavator (not shown). This is as should be understood by those skilled in the art. A first end (11) of the body (2) is formed to provide a first jaw (12). The first jaw (12) may include a locking system to secure a pin therein. The locking system is not shown to simplify the Figures. However it could be any known or yet to be developed locking system. A second end (13) of the body (2) has an aperture (14) into the cavity (5). The inside surfaces of side walls (3) have channels (15) one of which is shown in Figures 2A and 2B. Each channel (15) has a top surface (16) and a bottom surface (17). A row of recesses (18 - 20) in the bottom surface (17) are spaced apart along the length of 11 the channel (15). Recesses (18 - 20) provide a multi centred locking mechanism as should become clearer from the following description. The channels (15) define an axis of movement for the slide (6) allowing this to move forward and backwards freely. The axis of movement is shown as line (Y). The slide (6) has a jaw (21). The jaw (21) is the primary jaw of the coupler (1) as should be known to those skilled in the art. Slide (6) has guide portions (22). The guide portions (22) have a shape corresponding to channels (15). Therefore the guide portions (22) may be disposed in the channels (15). It should be appreciated that the channels (15) define a path to guide movement of the slide (6). The slide (6) has slot apertures (23). The slot apertures (23) can receive a connection pin (24). Locking members (25) are pivotally attached to the slide (6) at points (26A). The locking members (25) are shown in Figure 4. The locking members (25) have connection apertures (26). The axis of the connection apertures (26) is shown as line (X). Axis (x) is at a 45 degree angle to axis of movement (Y). The locking members (25) have a nub (27). The nub (27) provides a locking edge shown by line (28), and a leading edge shown by line (29). The leading edge (29) is shaped so that it does not hinder movement of the slide (6) towards the second end (13). The locking edge (28) is shaped so that it stops the slide (6) moving towards end (11) when in the locking position. Biasing elements (30) urge the locking members (25) to pivot around points (31). The biasing elements (30) may be 12 springs or rubber detents. Connection pin (24) extends through the slot apertures (23) and connection apertures (26). An actuator (32) in the form of a hydraulic cylinder is positioned inside the cavity (5). The actuator (32) is connected to a control system (not shown). The control system allows a user to control extension or contraction of the actuator (32). End (33) of the actuator (32) is secured to the body (2). End (34) of the actuator (32) is connected to the connection pin (24). The slot apertures (23) are approximately 20% longer than the diameter of the connection pin (24). This provides slack in the connection of the actuator (32) to the locking members (25). Extension of the actuator (32) moves the slide (6) forward (towards second end (13)). This will be referred to herein as locking movement. Contraction of the actuator (32) moves the slide (6) towards the first end (11). This will be referred to herein as releasing movement. The operation of the coupler (1) will now be described with reference to Figures 2A and 2B. The jaw (12) engages a first pin (36A) on a work attachment (neither shown in Figures 2A or 2B). The coupler (1) is rotated about the pin. The actuator (32) extends to move the slide (6) forward (towards end (13)). Biasing elements (30) urge locking members (25) towards a locking position. However, the leading edge (29) does not hinder movement of the slide (6) towards end (13). Movement of the slide (6) continues until the jaw (21) engages pin (36B) on a work 13 attachment (not shown). This secures the work attachment to the coupler (1). At this position the locking members (25) do not engage any of the recesses (18 - 20). The actuator (32) applies a driving or engagement force that ensures that the jaw (21) engages the pin (36B). The position of the recesses (18 - 20) is selected so that these correspond to the positions in which the jaw (21) engages a pin. That is, when the jaw (12) engages a pin (36B) the locking members (25) are adjacent to one of the recesses (18 - 20). Note that when the jaw (21) engages pin (36B) the nubs (27) do not align with a recess (18 -20). The work attachment can be used as per normal operation. If the actuator (32) loses hydraulic pressure the slide (6), and therefore jaw (21), moves along the length of the channels (15) towards end (11). However, this aligns each nub (27) with one of the recesses (18-20). The biasing elements (30) urge the locking members (25) to pivot and thereby force nubs (27) into one of the recesses (18 - 20). In the embodiment shown in Figure 2B this is recess (18). This is the locking position. The locking members (25) secure the slide (6) with respect to the body (2). This protects against loss of engagement force due to failure of the actuator (32). The connection of the actuator (32) to the slide (6) is such that the locking mechanism secures the slide (6) with respect to the body (2) until deliberate movement of the actuator (32) moves the locking members (25) to the release position. That is, to release the pin (36B) from the jaw (21) an operator sends a signal to the actuator (32) to contract. The 14 actuator (32) moves the connection pin (24) along the length of the slot apertures (23) towards end (11). The connection pin (24) presses against the edges of the connectionr apertures (26). The axis of the connection apertures (26) causes the connection pin (24) to move the locking members (25) thereby drawing the nubs (27) out of recesses (18) and moving the locking members (25) into the release position. In the release position the slide (6) can move with respect to the body (2) to release the pin (36) and thereby release the work attachment from the coupler (1). It should be appreciated that the use of multiple recesses (18-20) which are spaced along the channels (15) allows the locking mechanism to secure the slide jaw (21) at spaced apart positions along the length of the channels (15). This may be beneficial where the coupler (1) is used with work attachments (not shown) having pins (36A, 36B) of different spacings. Therefore, were the actuator (32) to fail then the recesses may facilitate a locking member (25) preventing the jaw (21) releasing the pin (36B). Therefore, the coupler (1) and locking mechanism guard against loss of engagement force and may facilitate a coupler being used with different types of, or specification, work implements. Referring now to Figures 5A - E showing an alternate embodiment of the coupler (1). Like numerals are used to refer to like components from Figures 1 - 4. The components of the coupler (1) are identical to that shown and discussed with reference to Figures 1 - 4. However, the orientation of the locking members (25) and recesses (18 20) has been altered. That is, the recesses (18 - 20) are now in the top surface (16) of the channel (15). The nubs (27) now face upwards towards top surface (16). The biasing elements (30) urge the locking members (25) to pivot upwards with respect to the slide (6). In all other aspects the operation of the coupler shown in Figures 5A - E is identical to that shown in Figures 1 15 -4. Figure 5A shows the coupler (1) having the actuator (32) fully contracted. This moves slide (6) so as jaw (21) releases pin (36B). Note that locking member (25) is rotated so that nub (27) does not engage or extend into one of the recesses (18 - 20). Figure 5B shows the actuator (32) partly through its stroke. The jaw (21) is moved towards pin (36B). Continued extension of the actuator (32) causes the jaw (21) to engage the pin (36B) as shown in Figure 5C. Note that locking members (25) have been moved past recesses (18 20). Nub (27) does not align with, nor extend into, any of recesses (18 - 20). If loss of engagement force occurs through failure of actuator (32) slide (6) can move with respect to body (2) to the position shown in Figure 5C. Note that locking member (25) has been moved along the length of path (15) so that nubs (27) align with recess (18). Biasing elements (30) force locking members (25) so as to pivot upwards towards top surface (16). This causes nubs (27) to extend into recesses (18). The locking member (25) prevents the slide (6) moving further towards end (2). Therefore, the jaw (21) does not fully release pin (36B). Accordingly, the locking mechanism prevents the coupler from releasing the work attachment. Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims. 16