CONTAINER STORAGE AND RETRIEVAL SYSTEM AND METHOD Technical Field This invention relates to a method and apparatus for storage in a warehouse environment of cases or other containers and to methods and apparatus for retrieval of containers. Throughout this specification and claims, the word "container" is used to refer to any suitable means for holding product and may include for example, crates, boxes, tins, cartons, cases, totes, bound together product or the like and also includes a plurality of grouped containers, such as a pallet of containers or a group of bound containers. Background The term "order picking" has become associated with systems designed for receiving, storing and delivering product to and from some form of storage area. They may also use some form of warehouse management system for co-ordination of storage. Products for distribution are often stored in a warehouse and retrieved therefrom for loading onto a vehicle for transport to customers. In an effort to increase the speed and efficiency of the storage and delivery system, apparatus for automated retrieval, or "picking", of product from the storage space have been developed. This has represented a large advance in the efficiency of order picking systems, which traditionally heavily relied on manual handling. Further advantages of automated systems include reduced overall cost, increased accuracy and decreased risk of personal injury. An ongoing problem faced by warehouse managers is the efficient use of space within a warehouse. Each square metre of floor space within the warehouse has an associated cost and the warehouse management system must seek to obtain the maximum use of the space in the warehouse to be efficient and competitive. Picking systems, whether manual or automatic, typically have a defined and fixed "pick face", or surface from which they can retrieve product. One problem presented to pick systems is how to replenish pick locations once they have been emptied. Traditionally, such replenishment is performed manually, with the assistance of a forklift or similar. This requires access to all parts of the storage area, the access channels, roads or similar requiring valuable space. Another problem faced by pick system designers is how to minimise the distance that the picking means, automated or manual has to travel to fulfil typical orders. The more the picking means has to travel, the longer the picking takes. 2 Due to demand variations, some products will invariably ship in greater quantities than others. Individual deliveries to customers may consist of relatively large quantities of a few products, but only a few, or even single cases or individual items of other products. This variability of product volumes presents a logistics problem in attempting to use the available resources, whether automated or manual or a combination most efficiently to obtain the best throughput. Furthermore, the product stored in a warehouse may include a substantial variety of any given product. For example, a warehouse storage for milk will include crates containing cartons or bottles of different capacity, different flavour (e.g. conventional, chocolate, strawberry, banana, etc) and of different constituents or nutritional composition (e.g. full fat, trim, super-trim, skim, high calcium, etc). In addition, the product may be sorted by date of production. Thus, a warehouse may contain a large range of product over a wide area. Selection of the product to fill specific orders is, consequently, a complex process requiring: a) a sophisticated warehouse management system for the location of product delivered and stored, and for the selection of product for an order; and b) an efficient system for access to and removal of product from the storage area to fill an order. Automated, robotic systems for order picking generally involve an x-y gantry system and a design for picking up individual containers or individual stacks of containers and transporting them from or to a conveyor. In the usual course, orders are delivered on pallets. Thus, the individually collected containers must then be formed into stacks of a required height, the stacks then formed into frames or partial frames of a required width and the frames or partial frames combined to form a pallet unit. Such systems can be inefficient and/or impractical in a large warehouse environment where orders require product to be collated from many different parts of the warehouse. The robotic pickers have to cover large distances, back and forth, in the warehouse to complete a given order. An existing automated storage and retrieval system is available from Automated Fork Truck Inc. of Salt Lake City, Utah, United States of America, This system is a storage and retrieval fork system that stores and retrieves product from vertically stacked racks. The system places product into and retrieves product from the racks through a vertical pick face at the end of a number of stacked racks and each rack being more than one pallet deep, with pallets being pushed away from the pick face for storage of another pallet in the same rack. With this system, the number of pick faces is limited and removal of individual containers from within pallets Is not facilitated. 3 us 6,061,607 discloses an order picking system for retrieving high volume and low volume product from two separate regions, but more specifically involves the location of product in vertical stacks in cells of totes. Low demand product is retrieved by a picker mechanism in a pick zone, by movement of that mechanism vertically above the pick zone and selection of individual articles from selected cells in selected totes. The system is primarily directed towards storage and retrieval of individual articles, which may be of high or low demand, rather than of containers containing a plurality of articles, which must, inevitably, be stored and transported in a different way. US 5,636,966 discloses a case picking system that removes full layers of cases and individual cases from storage towers. The storage towers are replenished from a further tower acting as a replenishment system. This requires double-handling of the transported layers. Moreover, the layers themselves are more demanding in their transport requirements than are individual cases or pallets. In order to maximise the efficiency of such a system, it is important that the largest "natural unit of handling" be used where ever possible. Typically one "natural unit of handling" may be a full stack of a defined number of containers. A larger natural unit of handing may be a defined number of full stacks, referred to herein as a "frame". The number of stacks in a frame, and the number of containers in a full stack, may be determined by the physical limitations of the bulk shipping and container handling apparatus. For example, the size of a frame of product, e.g. 6 full stacks, each 6 containers high, may be determined by the maximum number of containers which a specially equipped forklift can carry. Efficiency within the warehouse is promoted by moving and storing frames of product together wherever possible, and by moving full stacks of product wherever it is not possible to move entire frames, or where less than an entire frame is required. International Application No. PCT/NZ2002/000008 describes an order picking system whereby product is stored in high, medium and low demand zones, with high demand product being handled in frames, medium demand product being handled in full stacks and low demand product being handled in stacks or partial stacks. This system works well, but may not make optimum use of the space available. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in any country. Object of the Invention 4 It is an object of the present invention to provide a system for storing and/or retrieving containers that efficiently uses the available footprint and/or quickly stores and retrieves product, overcoming or alleviating problems with storage and retrieval systems at present. It Is an alternative object of the present invention to provide a method for storing and/or retrieving containers that efficiently uses the available footprint and/or quickly stores and retrieves product, overcoming or alleviating problems with storage and retrieval methods at present. It is an alternative object to at least provide the public with a useful choice. The above objects are to be read disjunctively. Further objects of the invention may become apparent from the following description, given by way of example only. Brief Summary of the Invention According to a first broad aspect of the present invention there is provided a system for storing and retrieving containers, the system comprising: ■ providing a first conveying means operable to move stacks of containers between a loading position and a discharge position; ■ a container storage area; ■ a container store dispatch conveyor; ■ a container store delivery conveyor; ■ first transfer means for transferring containers from the container store dispatch conveyor to the first conveying means; ■ second transfer means for transferring containers from the first conveying means to the container store delivery conveyor; ■ container store storage/retrieval means adapted to move containers from the container storage area to the container store dispatch conveyor and from the container store delivery conveyor to the container storage area; wherein ■ the first conveying means is provided with stack destacking means capable of destacking full stacks into at least two partial stacks, the stack destacking means positioned to allow containers which have passed through the destacking means to be delivered to the container store by the container store delivery conveyor. 5 Preferably the stack destacking means is a combined stacking/destacking means. Preferably, the stack destacking means is positioned to receive containers dispatched from the container storage area by the container store dispatch conveyor. Alternatively the system may be provided with a second stack destacking means positioned to receive containers dispatched from the container storage area by the container store dispatch conveyor. Preferably, the system includes a full stack delivery conveyor, adapted to transport full stacks of containers from the first conveying means to the container storage area. Preferably, the system includes a partial stack storage conveyor loop adapted to transport selected partial stacks to the container storage area. Preferably the partial stack storage conveyor loop includes a first partial stack storage conveyor and a second partial stack storage conveyor. Preferably the container store storage/retrieval means includes a crate robot. Preferably the crate robot is provided with a platform adapted to support at least one stack. Alternatively the crate robot is provided with a conveyor. Preferably the crate robot is adapted to engage and transport a plurality of stacks at once. Preferably crates are stored in the container storage area in rows of stacks, each row having a predetermined maximum number of stacks (N). Preferably the crate robot is adapted to engage and transport a number of stacks at once, the number being equal to or greater than N. Preferably the crate robot is adapted to support at least a full frame of stacks. Preferably the container store dispatch conveyor is adapted to support at least as many stacks as the crate robot. Preferably, the container store includes a plurality of rows of containers arranged in a first rank of rows. Preferably, the first rank is substantially parallel to the first conveying means. 6 Preferably, the container store includes a plurality of rows of containers arranged in a second rank of rows. Preferably, the second rank is substantially parallel to the first conveying means, and is provided between the first rank and the first conveying means. Preferably first rank and second rank are spaced apart to create an aisle therebetween. Preferably the crate robot is moveable along the aisle. According to a second broad aspect of the present invention there is provided a system for storing and retrieving containers, the system comprising: ■ first conveying means operable to move stacks of containers between a loading position and a discharge position; ■ a container storage area adjacent to the first conveying means in which containers are stored in rows of stacks, wherein the rows are substantially transverse to the first conveying means, and each row has a predetermined maximum number of stacks; ■ container loading means for transferring containers from the first conveying means to the container storage area; and ■ container retrieving means for transferring containers from the container storage area to the first conveying means. Preferably the container retrieving means can retrieve a predetermined maximum number of stacks in a single operation; wherein: ■ the predetermined number of stacks in at least one of the rows of the container storage area is greater than the maximum number of stacks which can be retrieved from container storage area by a single operation of the container retrieving means. Preferably the predetermined number of stacks in each row of the container storage area is greater than the maximum number of stacks which can be retrieved from container storage area by a single operation of the container retrieving means. Preferably the container retrieving means can retrieve a maximum of one full stack of containers per operation. Preferably the container storage area is provided with moving means for moving stacks of containers stored therein towards the first conveying means. 7 Preferably the container storage area is provided with parallel spaced-apart support members adapted to support a base the stacks in the row, and the container storage area is provided with one or more index conveyors engageable with one or more containers supported by the support members to transport it or them longitudinally, wherein said one or more index conveyors is movable transverse to said support members below said support member to enable selective engagement with containers positioned on said support members. Preferably the first conveying means includes a single endless conveyor. Alternatively the first conveying means includes a loading conveyor and an unloading conveyor substantially parallel and adjacent to the loading conveyor. Preferably the first conveying means includes a carriage. Preferably the carriage is adapted to carry one full frame of containers. Preferably the first conveying means is operable to convey stacks of containers from the loading position to the discharge position without handling by the container loading means or container retrieving means. Preferably the first conveying means includes spacing means adapted to create predetermined spaces between consecutive stacks of containers. Preferably the container loading means and container retrieving means are the same mechanism. Preferably the mechanism is a crate robot. Preferably the container retrieving means includes the crate robot and a container store dispatch conveyor. Preferably the system includes a control means adapted to receive an order. Preferably the control means determines a required number of each type of containers to be retrieved from the container storage area, and a required number of each type of containers to be moved from a bulk storage area to the first conveying means, to fulfil the order. Preferably the control means operates the container retrieving means to move the containers from the container storage area to the first conveying means. 8 Preferably the control means creates an output signal indicative of the containers to be moved from the bulk storage area to first conveying means. Preferably the control system monitors the position of each container in the container storage area. Preferably the system includes means for determining the presence of vacant spaces on the first conveying means which are sufficiently large to accommodate a stack of containers. According to a third broad aspect of the present invention there is provided a method of storing and retrieving containers, the method comprising: ■ providing a first conveying means operable to move stacks of containers between a loading position and a discharge position; ■ storing containers in a container storage area which is adjacent to the first conveying means; ■ providing container loading means for transferring containers from the first conveying means to the container storage area; ■ providing container retrieving means for transferring containers from the container storage area to the first conveying means, wherein the container retrieving means can retrieve a predetermined maximum number of stacks in a single operation; wherein ■ the method further includes storing the containers in the container storage area in rows of stacks which are substantially transverse to the first conveying means, wherein at least one of the rows has a predetermined maximum number of stacks which is greater than the maximum number of stacks which the container retrieving means can retrieve in a single operation. According to a fourth broad aspect of the present invention there is provided a method of storing and retrieving containers, the method comprising: ■ providing a first conveying means operable to move stacks of containers between a loading position and a discharge position; ■ transferring stacks or partial stacks of containers from the first conveying means to a container storage area, and/or from the container storage area to the first conveying means, as required to supply a required number of containers to the discharge position; and ■ the method further including destacking selected stacks of containers which are being moved by the first conveying means into first and second partial stacks, and moving one 9 of the partial stacks to the container storage area, If necessary to provide the required number of containers to the discharge position. Preferably the method includes moving whole stacks from the first conveying means to the container storage area with a full stack delivery conveyor. Preferably the method includes moving partial stacks from the first conveying means to the container storage area with a remainder conveyor, which is separate from the full stack delivery conveyor. Preferably the method includes storing selected partial stacks on a partial stack storage conveyor loop. According to a fifth broad aspect of the invention there is provided a method of operating a container storage and retrieval system comprising a bulk storage area, a container storage area and a dispatch area, the method including receiving an order for one or more types of containers, and for each type of container: • Calculating the required number of containers (N) divided by the number of containers in a frame (F) to obtain a quotient (Q) and a remainder (R); • Moving Q frames from the bulk storage area to the dispatch area; • Determining the number (C) of the required type of containers in the container storage area; and • If CsR, moving R containers to the dispatch area; or • If CR, moving R containers to the dispatch area; or V. If C