PORTABLE LIQUID DISPENSING APPARATUS FIELD OF THE INVENTION The present invention relates generally to the dispensing of water and other liquids, and in particular to a portable apparatus that may be used to contain and dispense water and other liquids for household cleaning and similar applications. BACKGROUND OF THE INVENTION It is well known that the use of ordinary garden hoses for outdoor cleaning tasks, such as washing vehicles, generally results in a high level of water wastage. In order to mitigate this drawback, high pressure cleaners have been developed to perform these tasks effectively while using smaller quantities of water. As a result of their improved water efficiency, in many areas it remains permissible for people to use high pressure cleaners, either attached to domestic mains water supplies, or in commercial car washes, even during periods of water restrictions due to drought or other causes of declining water reserves. Commercially available high pressure cleaners provide flow rates typically ranging between 6.5 and 10 litres per minute, and may provide pressure in excess of 1 ,450 psi (10 MPa). Accordingly, as an example, a low flow rate high pressure cleaner operating for 10 minutes during the process of washing a vehicle will consume at least 65 litres of water. While this represents a relatively low water usage, high pressure cleaners are not ideal for this task, and have a number of disadvantages. Firstly, the quality of wash achieved using a high pressure cleaner may often be inferior to that which can be achieved using the combination of a bucket and/or hose and a sponge. Furthermore, the very high output water pressure may result in damage to the exterior of the vehicle being washed, including the removal of, or damage to, trimmings and accessories such as decals, pin striping, headlight and bonnet protectors, metal trim and even paint. Additionally, door and window seals are not generally designed to withstand the direct application of a high pressure jet, and therefore in some cases the use of high pressure cleaners can result in significant quantities of water leaking into the interior of the vehicle. Yet another problem of high pressure cleaners is that they typically require not only a water supply, but also a mains power supply, which may not be conveniently available. Of course, the combination of mains power with water, particularly when ad hoc supply solutions are employed, such as extension cords, raises significant safety concerns. The use of a bucket and sponge rather than a high pressure cleaner may result in a superior wash quality, and largely eliminate damage to vehicles and trimmings, as well as involving no inherent risk of electric shock. Nonetheless, many people find the use of a bucket and sponge to be inconvenient and excessively time consuming. This is particularly the case in relation to rinsing of a vehicle after washing with water and detergent. For an average sized vehicle, if a standard 10 litre bucket is used to transport water for rinsing, each bucket load typically provides around nine litres of rinsing water. For an average sized vehicle, it is reasonable to assume that one bucket load will be required for rinsing the roof and upper surfaces of the car, a further four bucket loads for rinsing the sides/comers of the vehicle, and typically an additional bucket load for a final rinse of the entire vehicle. Assuming that a further bucket load of water is used with detergent for washing of the vehicle, the total water usage is comparable to that of the high pressure cleaner, however it is likely that a superior wash quality will be obtained. As will be appreciated, however, the main drawback of the method is the need to transport a number of heavy buckets of water to the vehicle, and to lift the full bucket over the vehicle in order to perform the rinsing task. In this regard, it is particularly difficult using a bucket to rinse the roof of a vehicle due to its height and to rinse the sides of the vehicle due to difficulties in directing the water effectively at the areas to be rinsed. Many people consider it more convenient to use a large watering can rather than a bucket, however, this does not significantly reduce the problems associated with the weight of the water, especially when attempting to rinse the roof of the vehicle. Similar problems arise when using a bucket or watering can to water household or outdoor plants. Again, it may frequently be necessary to raise a heavy bucket or watering can of water to a suitable height and direct the flow of water accurately in order to water a plant. Nonetheless, it is increasingly common for people to water the plants in this way, particularly in order to use "recycled" or "grey" water, such as used bath or laundry water, as a means of conserving water and/or minimising the environmental impact of water usage. It will be appreciated that there may be many other household and industrial uses of buckets for transporting and/or containing water or other liquids in which the bucket, while useful and reasonably effective, is not as easy or convenient to use as may be desirable. Furthermore, it is believed by the present inventor that further reductions in water usage in cleaning tasks are achievable, over those that are obtained through the use of high pressure cleaners, buckets and/or watering cans. Accordingly, it would be highly desirable to provide an improved form of dispensing apparatus that may result in reduced water usage and/or that is more convenient, efficient and/or useful for a variety of household and/or industrial applications such as those discussed in the foregoing paragraphs. It will be appreciated that any discussion of documents, devices, acts or knowledge in the specification is included to explain the context of the invention. Accordingly, such discussion should not be taken as an admission that any of the material formed part of the prior base or the common general knowledge in the relevant art on or before the priority date of any of the statements included herein. SUMMARY OF THE INVENTION The present invention provides a portable liquid dispensing apparatus suitable for household use including: a self-contained supply compartment for containing a liquid such as water, cleaning fluid or the like; and a relatively low pressure motorised pump having an input conduit opening into the supply compartment to receive liquid therefrom, and an output conduit which is connected in use to a dispensing assembly that is arranged to receive liquid pumped from the supply compartment by the pump, and to produce an output flow of liquid that may be directed by a user of the dispensing apparatus. Accordingly, the invention enables a user of the apparatus to transport water or other liquid contained in the supply compartment without need of a connection to the water mains, in much the same manner as a conventional bucket or watering can, while providing for improved ease of dispensing and use of the liquid contents. Furthermore, the use of a relatively low pressure pump, which may be capable, for example, of pumping liquid at a rate of up to about three litres per minute, may result in significantly reduced water usage in cleaning applications as compared with high pressure cleaners or conventional buckets. In this regard, it will also be understood that the operating pressure of the pump is relatively low by comparison with commercially available high pressure cleaners, so as to mitigate or avoid their associated disadvantages, such as damage to vehicles, trimmings or accessories and/or the leakage of water into a vehicle via door and window seals. For example, a suitable pump operating pressure may be less than 150 psi (1 MPa), and indeed an operating pressure of less than 50 psi (350 kPa), similar to that available from typical automotive windscreen washer pumps, may provide satisfactory results. Advantageously, the dispensing assembly enables the user to direct the resulting flow of liquid towards a desired target, such as an area of the surface of a vehicle to be washed or rinsed. This degree of control over the direction of flow of the liquid may enable the volume of water used in cleaning applications to be further reduced. It will be appreciated that the ability to easily control the direction of flow of liquid dispensed from the apparatus, without the necessity to lift and/or tip a bucket or watering can may also provide improved utility in other applications, such as watering plants, washing and/or rinsing a variety of household items, and indeed any application in which a directed flow of relatively small quantities of liquid at relatively low pressure is required. In preferred embodiments the input conduit of the motorised pump opens into the supply compartment in a region close to the bottom of the supply compartment. This arrangement ensures that the majority of the contents of the supply compartment can be pumped out, thereby maximising the ability of the user to dispense the liquid contents of the compartment. It is further preferred that the motorised pump is powered by a battery, it being particularly preferred that the battery is mounted in a substantially waterproof battery compartment within the apparatus. Advantageously, in preferred embodiments the apparatus therefore does not require the use of a mains power supply, resulting in a substantial mitigation of the potential risks associated with the use of mains power in a wet environment. As will be appreciated, the use of mains power, in combination with suitable waterproofing, insulating, and other safety features, is not precluded however. The dispensing assembly preferably includes a fluid delivery device connected to the output conduit of the pump by a flexible supply line, such as a hose made from rubber or other flexible material. In preferred embodiments, the fluid delivery device is intended to be hand held by the user of the apparatus. It is therefore particularly convenient for the fluid delivery device to be a pistol grip style device. The dispensing assembly may also include an electrical connection to the battery and/or pump, and a switch for activating and deactivating the pump under the control of the user. As will be appreciated, by providing an electrical connection, and a switch on the hand held fluid delivery device, the user is provided with control over the operation of the pump, which may be turned on and off without the user having to bend down to operate a switch that otherwise may be provided on the body of the apparatus. It is further preferred that the dispensing assembly includes a flow control valve operable to control the rate of flow of liquid pumped from the supply compartment through the dispensing assembly. Again, it is particularly convenient to locate the flow control valve within the hand held fluid delivery device for ease of operation by the user. While the fluid delivery device may include a single output channel for delivery of fluid to a single outlet, it is preferred that the fluid delivery device includes multiple output channels and/or is able to deliver fluid to multiple outlets. Accordingly, in a particularly preferred embodiment, the fluid delivery device includes first and second output channels, and a flow direction valve operable to direct the flow of fluid pumped from the supply compartment to either one of said output channels. In the preferred embodiment, the first output channel directs fluid to a tip of the fluid delivery device for providing a generally forwardly directed fluid output. The second output channel preferably directs fluid to one or more outlets, such as spray jets, located along one or more sides of the fluid delivery device for providing a generally sideways and/or vertically directed fluid output. It will be appreciated, for example, that such an alternative sideways directed fluid output may be particularly useful when rinsing articles or surfaces, such as the roof of a vehicle, towards which it may be difficult to direct the fluid flow from a forwardly directed fluid output. The tip of the fluid delivery device may include a spray jet. In preferred embodiments, the tip includes an interchangeable attachment Accordingly, for example, the spray jet may be replaced with alternative outlet devices, such as an extension adaptor for providing an extended reach to the user in directing the output flow of liquid. It will be appreciated that various other interchangeable attachments may be provided such as, for example, sprinkler attachments to assist in the watering of household and garden plants using the apparatus. The supply compartment may be an enclosed compartment having a removable closure to enable the compartment to be filled with a liquid. Preferably the removable closure is a vented cap, to enable a pressure balance to be maintained between the enclosed compartment and the outside atmosphere as the fluid is dispensed. In a particularly preferred embodiment, the apparatus further includes a trough having an open top. Advantageously, such an open trough, in a manner similar to a conventional bucket, may contain, for example, a cleaning fluid such as water and detergent. The contents of the trough are easily accessible, for example when it is desired to apply the cleaning fluid to a surface using a sponge through the open top of the trough. In preferred embodiments, the trough is located over the supply compartment such that the enclosed supply compartment extends beneath and around the trough. In this arrangement, a removable plug may also be provided in the base of the trough such that, when the plug is removed, liquid may pass between the trough and the supply compartment. Accordingly, this configuration enables the liquid holding capacity of the trough and the supply compartment to be combined, to provide for an increased total supply capacity of the apparatus. The apparatus may also include a removable grit screen, positionable in the base of the trough, which may be, for example, a wire or plastic mesh. Where provided, the function of the grit screen would be to block the passage of particles of grit between the lower portion of the trough and the main body of the trough. On the one hand, therefore, the grit screen may be used to prevent particles of grit that enter through the open top of the trough from passing into the enclosed supply compartment when the removable plug is not installed. On the other hand, when the removable plug is in place, sealing the trough from the supply compartment, the grit screen may be used to prevent grit that has gathered in the bottom of the trough from passing into the main body of the trough, from where it may be picked by a sponge or other cleaning cloth and thereby cause damage to the surface of an object being cleaned. The motorised pump may be reversible, such that liquid may be drawn into the supply compartment through the input conduit in a reverse mode of operation. While liquid may be drawn through the dispensing assembly, it is preferred that an appropriate alternative supply line be provided, to enable an increased inflow rate to be achieved, as compared with the rate of dispensing in the forward mode of operation. It is further particularly preferred that the apparatus include wheels or castors mounted at the base to enable the apparatus to be moved about using said wheels or castors. Advantageously, this enables the apparatus to be easily transported from a source of water or other liquid to the intended location of use, without the need for the user to lift a heavy bucket when full. BRIEF DESCRIPTION OF THE DRAWINGS Further preferred features and advantages of the present invention will be apparent to those skilled in the art from the following description of a preferred embodiment of the invention. It will be understood, however, that the preferred embodiment is not limiting of the scope of the invention as defined in any of the preceding statements. The description makes reference to the accompanying drawing, in which Figure 1 is a schematic diagram, shown in cross section, of the preferred embodiment of the apparatus of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Figure 1 illustrates schematically a preferred embodiment of a portable liquid dispensing apparatus 100 in accordance with the invention. The apparatus 100 includes a main body part 102 which, in the preferred embodiment, is constructed around moulded casing 104. The casing 104 may be, for example, blow moulded from plastic in any desired shape or design, and it will be appreciated that the particular configuration shown in the preferred embodiment 100 is a convenient, but only exemplary, arrangement. The casing 104 includes a supply compartment 106, which is a closed compartment that forms an inner bucket of the portable apparatus 100. A separate, and preferably substantially waterproof, compartment 107 houses a motorised pump 108. The pump has an input conduit 110 which opens into the inner bucket 106, and an output conduit 112. Accordingly, when the pump is operating it serves to pump any liquid such as water, contained within inner bucket 106 to the output conduit 112. The pump is designed to operate at a relatively low pressure as compared with the pressure provided by known high pressure cleaning devices. The apparatus 100 further includes a dispensing assembly including, in the preferred embodiment, flexible supply line 114 and fluid delivery device 116. The flexible supply line 114 may be, for example, a rubber or flexible plastic hose. In the preferred embodiment, the fluid delivery device 116 is a pistol grip style device, for easy handling by a user of the apparatus. In use, as water or other liquid contained within the inner bucket 106 is pumped out by pump 108, the internal fluid level will drop. Accordingly, the closed compartment of the inner bucket 106 is provided with vented cap 117 disposed at the top of the compartment. The cap is removable, to enable the user to fill the compartment with water or other liquid. The vent is provided to enable a pressure balance to be maintained between the enclosed inner bucket compartment 106 and the outside atmosphere as the contents of the compartment 106 are pumped out. To avoid safety hazards associated with the use of mains power within a wet environment, in the preferred embodiment the pump 108 is powered by battery 118, which is held within further compartment 119, which again is preferably substantially waterproof in order to protect the battery 118. Electrical connections are provided between the terminals of the battery and corresponding supply terminals of the pump. In the preferred embodiment, this circuit includes electrical connections 120 between battery 118 and pump 108. The electrical connections 120 connect to an actuating switch 122 mounted in the pistol grip delivery device 116. Accordingly, the circuit is completed to cause the pump to operate by closure of actuating switch 122. By virtue of the location of switch 122 on the pistol grip delivery device 116, the user is able to control operation of the pump 108 directly using the hand held delivery device 116, and without the necessity to bend down and operate a switch that may otherwise be provided on the main body 102 of the apparatus. The hand held delivery device 116 also includes flow control valve 124, that enables the user to control the flow rate of fluid pumped from the inner bucket compartment 106 by pump 108. Again, the benefit of including the flow control valve 124 within the delivery device 116 is that the user is easily able to operate the valve while directing fluid pumped from the compartment 106 in the desired manner. As a further feature, the preferred embodiment includes within the delivery device 116 two separate output channels 126, 128. A flow direction valve 130 provided on the delivery device 116 allows the user to control the flow of fluid into either one of the output channels 126, 128. The first output channel 126 directs fluid to horizontal spray jet 132, which in use results in the dispensed fluid being directed in a substantially forward direction relative to the delivery device 116. The second output channel 128 directs fluid to the three vertical spray jets 134, which in use provides for a substantially sideways or vertically directed fluid output relative to the delivery device 116, depending upon the orientation with which the device 116 is held by the user. As will be appreciated, horizontal spray jet 132 is useful to direct fluid directly onto surfaces or articles that are easily directly accessible. For example, horizontal spray jet 132 may be effectively used for rinsing the sides of a vehicle being washed using the apparatus, or onto other articles, items or surfaces located at a low level relative to the user. The vertical spray jets 134, on the other hand, be useful in directing the flow of liquid onto less easily accessible surfaces, such as the roof of a vehicle being washed using the apparatus. A further extension accessory 136 is also provided in the preferred embodiment, which may be connected in place of horizontal spray jet tip 132 to the forward end of the delivery device 116. As shown, the extension accessory not only extends the reach of the delivery device 116, but may also include a right angle bend 137 for directing the output flow. Such an extension accessory may, for example, be useful in rinsing particularly high surfaces, or for watering plants, such as those displayed in hanging baskets, that may otherwise be difficult to access using a watering can or bucket. The preferred embodiment of the liquid dispensing apparatus also includes, within the body part 102, an open topped trough 138, which effectively forms an outer bucket. As shown, the outer bucket is arranged over the inner bucket 106, such that the enclosed inner compartment 106 extends beneath and around the trough of the outer bucket 138. Removable plug 140 is provided in the base of the outer bucket such that, when the plug is removed, liquid may pass between the outer and inner buckets. Accordingly, with the plug removed the total supply capacity to pump 108 is effectively equal to the sum of the capacities of both the inner bucket 106 and the outer bucket 138. A removable grit screen 142 is also provided, which is locatable within the base of the outer bucket trough 138. The grit screen 142 is, for example, a wire or plastic mesh, which prevents the passage of sufficiently large particles of grit between the main body of the outer bucket 138 and the grit pit 144 formed beneath the screen 142. In different applications, the function of the grit screen 144 may be primarily to prevent passage of particles either from the outer bucket 138 to the inner bucket 106, when the plug 140 is removed, or to prevent the passage of trapped grit within the grit pit 144 into the main body of the outer bucket 138 when the plug 140 is installed. As will be appreciated, when the plug 140 is not present, it is desirable to prevent grit entering the outer bucket 138 from reaching the inner bucket 106, from which it may be difficult to remove and may ultimately cause damage to pump 108. Different concerns arise when the plug 140 is in place, as may be the case, for example, when the outer bucket 138 contains cleaning fluid such as water and detergent for washing an article such as a vehicle, and the inner bucket 106 contains water for use in rinsing the vehicle using the delivery device 116. In this situation, it is desirable to prevent trapped grit from moving within the outer bucket 138 from which it may be picked up on a sponge or other cleaning cloth and cause scratching or other damage to the surface of the bucket itself, or the surfaces of the vehicle being cleaned. While the main body 102 of the apparatus 100 may be formed in any desired shape or size, it is preferred that the maximum handling capacity of the combined inner bucket 106 and outer bucket 138 is up to about 20 litres. The weight of 20 litres of water is about the most weight that a typical user of the bucket is likely to be able to comfortably and safely carry over any given distance. In the preferred embodiment shown in the drawing, the total capacity is divided between approximately 9 litres in the outer bucket 138, and approximately 11 litres in the inner bucket 106. Even where the total capacity of the apparatus is limited to a reasonable volume of liquid, it is nonetheless desirable that the user not be required to lift and carry the apparatus if this can possibly be avoided. Accordingly, wheels or castors 146 are provided at the base of the main body 102 to enable the apparatus to be wheeled across substantially flat surfaces to avoid the necessity for lifting and carrying of the heavy apparatus when filled with liquid. A particular advantage of the apparatus 100 is that it enables the user to accurately and effectively deliver the liquid contained within compartments 106 with minimum wastage, and without resort to potentially harmful high pressure cleaners. For example, when washing a typical vehicle such as a family car, the relatively low pressure delivery, combined with the high precision with which the apparatus 100 enables rinsing water to be directed on to the surfaces to be rinsed, substantially reduces the amount of water that may be required. Accordingly, as little as 14 litres of water may be sufficient to completely wash and rinse a typical family car. Compared with an estimated 60 to 70 litres of water required using a high pressure cleaner, or a conventional bucket, this represents a substantial potential water saving. Indeed, assuming a weekly wash, over the course of a year the use of the present invention in its preferred form may require only 728 litres of rinsing water to be used, as compared with over 3,000 litres per year for conventional high pressure cleaners or buckets. The substantial saving in water results from the use of the on-board low pressure pump 108, which is able to deliver variable flow rates of up to about 3 litres per minute, and which enables the entire average car to be rinsed using as little as 5 litres of water. The remaining 9 litres used in the washing process is the water with detergent that is contained within the outer bucket 138, and which is applied using a sponge or other cloth in order to clean the surfaces of the vehicle. As another application, the apparatus 100 may be filled with used "grey" water from a washing machine or bath, which may then be used for watering of household or garden plants. Further useful and practical features may be incorporated into the apparatus 100. For example, it is preferable to provide for automatic shutdown of the pump 108 upon emptying of the inner bucket compartment 106. Such automatic shutdown prevents consequent damage to the pump 108 due to overheating in the absence of water or other fluid to be pumped through from inlet 110 to outlet 112. Simply re-filling the inner compartment results in a re-set of the shutdown mechanism. Furthermore, by allowing the actuating switch 112 to be fixed in the "on" position without continuous applied pressure by the user, the apparatus 100 may be left unattended while delivering its contents via the fluid delivery device 116. This is useful, for example, when using the device to water a particular region of a garden. In combination with an automatic shutdown feature, this is a perfectly safe and convenient use for the apparatus 100. Although not shown in Figure 1 , in some embodiments of the invention the pump 108 may be reversible, to enable liquids to be drawn into the supply compartment 106 through the input conduit 110. As will be appreciated, reversing the direction of operation of the pump 108 may be particularly straightforward in the case of a DC motorised pump, such as pump 108 illustrated in the drawing. In this case, simply reversing the polarity of the supply voltage provided from battery 118 reverses the pumping direction. A suitable switching arrangement may thus be readily provided, either on the body 102 of the apparatus 100, or on the hand-held delivery device 116. When operating in a reverse-pumping mode, liquid may be drawn into the supply compartment 106 via the dispensing assembly. However, since the dispensing assembly is designed to provide a relatively low flow rate, it is preferable that an appropriate alternative supply line be provided, which may be, for example, interchangeable with the dispensing assembly, allowing a higher rate of flow of liquids into the compartment 106. For example, a simple hose may be suitable for this purpose, although in some circumstances it may be desirable to fit a filter within the hose in order to restrict the entry of grit particles or other impurities through the pump 108 and into the supply compartment 106. Jt will be appreciated that the foregoing description of a preferred embodiment, and applications thereof, is exemplary only, and the scope of the invention will be understood to encompass variations, modifications and equivalents that would be apparent to persons of skill in the art. Furthermore, many other possible applications of the invention will be apparent. The invention should therefore not be considered to be limited by the preferred embodiment, or by the exemplary applications disclosed herein.