TITLE "SELF SCANNING SHOPPING SYSTEM AND METHOD" FIELD OF THE INVENTION The present invention relates to improvements to systems in which items to be purchased by a customer are scanned by the customer. BACKGROUND OF THE INVENTION Present shopping systems generally involve the process whereby a customer goes about the store, selecting the goods they desire to purchase and placing these goods in a trolley or basket. The collected goods are then taken to a checkout, where each item is individually scanned by the checkout operator, whereupon the customer tenders payment for the selected goods. Systems have been proposed whereby the customer performs the scanning operation at the checkout rather than having a staff member to perform this task. While this may decrease the costs to the store, it is likely that the time that it takes to scan and pay for the goods will not significantly reduce as the people doing the scanning may not be particularly familiar with the process or equipment. It has also been proposed to have a system in which a device including a scanner is provided on or near the shopping trolley so that the customer scans the goods as they are placed into the trolley. The device stores information regarding the goods that have been scanned and passes this information to the point of sale system at the checkout as the person leaves the store so that payment can be arranged. These types of system have the disadvantage that they must be able to interface with the point of sale system that the store is using therefore requiring specific design for different point of sale systems. Also, there are problems with ensuring that the goods that have been scanned are the goods in the trolley at the time when the customer leaves the store. The present invention attempts to overcome at least in part the aforementioned disadvantages of previous shopping systems. SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention there is provided a self scanning shopping system comprising: a portable scanning means being arranged to be carried around a store by a customer, the portable scanning means being able to scan the barcode on products in the store; a data storage means associated with the portable scanning means, the data storage means being arranged to store information including the barcodes of the products scanned by the portable scanning means; and an interface means arranged to receive the stored information from the .data storage means of the portable scanning means and to transmit barcode information to a point of sale system in a format in which the point of sale system would expect barcode information from a barcode scanning device. In accordance with a second aspect of the present invention, there is provided a method for determining when products scanned by a customer are to be rescanned for checking purposes comprising the steps of: assigning to each product one or more product values associated with characteristics of the product, said characteristics indicating the likelihood of the product being stolen; assigning to each customer one or more customer values associated 'with known characteristics of the customer, if any, said customer values being used to indicate whether the customer is a frequent customer of the store; and calculating using the or each product values and the or each customer values one or more sampling values, the or each sampling values being used to determine whether a particular product of a particular customer should be re-scanned. DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a self scanning shopping system in accordance with the first aspect of the present invention. DESCRIPTION OF THE INVENTION The present invention relates to a system for self scanning shopping 10 including a portable scanning means 12. The portable scanning 12 means may consist of a portable terminal, which is provided with an integrated barcode scanner. The portable scanning means 12 may be provided as a handheld unit or it may be provided as an attachment to a shopping trolley 18 or basket. The portable scanning means 12 is arranged to be carried around the store by the customer and used to scan products which the customer intends to purchase and places in their shopping trolley 18 or basket. The portable scanning means 12 is provided with an associated data storage means 14 within the portable terminal. The data storage means 14 is provided with a memory area in which information can be stored, the information including the barcodes of items which have been scanned by the current customer. Known devices are available which would fulfil the function of the portable scanning means 12. For example, consumer memory scanners are available which comprise a portable device which can scan and store barcodes. These devices are available with a cradle which receives the portable device, the cradle being connectable to a computer, to which the stored barcodes are transferred. When the customer has completed selection of their products and all products that are scannable have been scanned and placed in the trolley, the customer proceeds to the checkout. At the checkout is provided an interface means 16 which is arranged to provide information into the point of sale system 20 of the store. The interface means 16 may comprise a computer with suitable software or other similar device. The portable scanning means 12 is arranged to provide to the interface means 16 information including the barcodes of the products which have been scanned by the customer. The portable scanning means 12 may communicate with the interface means 16 via a cradle 15 arranged to receive and communicate with the portable scanning means 12 attached to a communication port on the interface means 16. Alternatively, the portable scanning means 12 may be connected remotely, for example via an infra-red or radio transmission to the interface means 16. The interface means 16 is connected to the point of sale system 20 in the same way in which a checkout barcode scanner would be connected if a barcode scanner were provided at the checkout in a known manner. The interface means 16 is arranged to provide barcode information to the point of sale system in a format which the point of sale system 20 would expect to receive the barcode information from the checkout barcode scanner. Commercial barcode scanners generally utilise either a serial or PS/2 output. The interface means 16 would therefore be configured to support both serial and PS/2 outputs. As the system of the present is designed to be integrated with existing systems, it is necessary to have the interface mans 16 properly configured to provide barcode data to the point of sale system in a format which the point of sale system 20 is currently receiving. If the barcode scanner is using a serial connection, it will be necessary to configure the interface means 16 to use the same data transfer characteristics (Baud, parity, Data bits, stop bits, handshaking protocol) and the same barcode format. For PS/2 connection, only the barcode format need be set. The self scanning shopping system 10 is provided also with a barcode scanner output interrogation device (not shown). The barcode scanner output interrogation device is connected to the existing barcode scanner and a number of known barcodes are scanned. The barcode scanner output interrogation device stores the data received from the barcode scanner and processes this data to determine the data transfer characteristics and barcode format which are then configured into the interface means 16 such that the interface means can duplicate them. The barcode scanner output interrogation device may be a separate electronic device or may be incorporated into the interface means 16. Once the interface means 16 is configured, the barcode data received from the portable scanning means 12 can be transferred to the point of sale system 20 in the same format as items scanned individually by a barcode scanner. That is, the barcode information for each product is transmitted in turn as if it has been scanned manually at the point of sale. This arrangement allows the self scanning shopping system 10 to be installed to work with any point of sale system 20 without the need to modify the point of sale system 20. The self scanning shopping system 10 may also be provided with a- security tag deactivation means (not shown). In some stores, items to be purchased are provided with security tag attached thereto. The security tags are arranged such that they can be detected if the product is taken from the store without the security tag being deactivated. Such security tags are usually deactivated at the time of scanning at the checkout. As the present system involves scanning as the items are placed in the trolley, the security tag deactivation means may be arranged adjacent the trolley at the checkout such that all of the security tags which are in the trolley and have been scanned can be deactivated before the customer leaves the store. To ensure the integrity of the present system, the system is provided with one or more auditing means. The auditing means may include a weight calculation means and a scale 30. The weight calculation means may be provided in the form of software and a processing means which calculates, from information gathered about the products at the time when the products are scanned, what is the total expected weight of the scanned products. The scale 30 is provided at the checkout to measure the total weight of the trolley 18 and goods, from which the known weight of the trolley can be subtracted. The trolley 18 may be provided with a suitable identification which can be scanned by the portable terminal to determine the weight of the trolley. The weight measured by the scale 30 is then compared with the calculated total expected weight of the products. If the two values are the same, given a predetermined margin of error, then the check is passed, otherwise the products must be rescanned. In a preferred embodiment, the scale 30 is provided in an aisle of the checkout in the form of a weighbridge. In this configuration, the aisle of the point of sale is provided with a pair of grooves that are arranged to receive the wheels of the trolley which then rest on the weighbridge. The grooves may further be provided with a locking means which detains the trolley in the aisle of the checkout until payment for the selected products has been tendered. Alternatively, a signal means, such as a light or sound, may be provided to indicate payment has been tendered and the procedures of the auditing means has been completed. A bridge member is provided substantially over and between the grooves 36 such that the customer may stand adjacent to their trolley without affecting the measurement of weight by the scales 30. The auditing means may be further provided with a secondary auditing means which provides a random check upon the products selected by the customer in order to control losses that may occur due to a product of relatively low value being substituted in the trolley by a product of comparatively high value of the same weight. The secondary auditing means operates on the basis of a mathematical formula that takes into account various factors pertaining to the product itself and the customer. Using this formula, products are selected for rescanning. The measure used by the secondary auditing means is allocated as the item sampling value (ISV). This value is determined from three factors. These factors include the weight of the item (IW) which is the weight of the item in grams, the item sampling factor (ISF) and the customer sampling factor (CSF). The item sampling factor is further comprised of the item value index (IVI) multiplied by the item risk factor (IRF). The item value index is the nominal item price, preferably in cents, divided by the item weight, preferably in grams. If no item value index is given, a value of 1.0 will be assumed. The customer sampling factor is further comprised of a customer value-index (CVI) multiplied by a customer risk factor (CRF). The customer value index is a value of one divided by the number of customer purchases per month. Casual customers may be allocated a nominal value of 1.0. The customer risk value may be a number value, preferably between 0.001 and 1000, which may denote the fraud risk of the customer. A nominal value of 1.0 may be allocated to casual customers. The item sampling value is thus defined by: Item Sampling Value = Item Weight x Item Sampling Factor x Customer Sampling Factor, or ISV = IW X ISF X CSF. In complete terms, ISV=IW x (IVI x IRF) x (CVI x CRF)

The item supply value may be used to determine when items are to be rescanned for checking purposes. In use, a customer is provided with a portable scanning means 12, preferably as a portable terminal upon entrance to the retail outlet. The customer may then use the portable scanning means 12 to scan the unique identification code of the trolley or basket they will be using to deposit the selected products they wish to purchase. Upon selecting each product, the customer scans the product with the portable scanning means 12 and the barcode information is stored in the data storage means 14. Successful scanning of selected products may be confirmed by a marking means that may individually mark each successfully scanned item. The marking means may comprise any suitable method of labelling a product, including visible or ultra-violet inks. Preferably, the labelling of the products by the marking means should be readily removable from the products to accommodate the removal of products from the selection made by the customer. Once the customer has selected and scanned their complete selection of products, the customer makes their way to the checkout. In a preferred embodiment of the present invention, the customer moves their trolley into the aisle of the checkout where the wheels of the trolley may be received by the grooves. The trolley may then be detained in this position until the procedures of the auditing means are completed. The portable scanning means 12 may then communicate with the interface means 16 which in turn provides barcode information to the point of sale system. Upon successful completion of the procedures of the auditing means, the point of sale terminal displays the amount that is to be tendered for the goods on a display means of the point of sale terminal. The customer may then tender payment for their selected products by any suitable means. Upon verification of the integrity of the selected products by the auditing means and payment therefor by the customer, the locking means may be released, allowing the trolley to depart from the aisle and from the retail outlet. Alternatively, the signal means indicates to the customer that the processes of payment and of the auditing means have been completed and that they may depart from the retail outlet. Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.