Hot Water Supply System Technical field The present invention relates to a hot water supply system (HWSS), in particular a hot water supply system for use with a heat pump that uses a thermal energy storage arrangement. Background Domestic and small commercial premises often meet requirements for hot water by a gas-fired boiler or furnace. Either the water is heated and stored in a tank of some kind or is heated on- demand when a hot tap (faucet) is opened on the premises. With increasing concern over the environmental damage caused by burning fossil fuels, attention has turned to alternative techniques to provide hot water, such as heat pumps. Heat pumps use refrigeration technology to extract heat from a lower-temperature resource and transfer that heat to a higher-temperature resource. A heat pump can, for example, extract heat from air at, say 15 degrees centigrade, and heat water to a temperature of, say 50 degrees centigrade. Such a heat pump is called an air-source heat pump although water-source and ground- source heat pumps are also available and work on the same principle. Despite the (usually electrical) energy required to operate them, heat pumps can be up to four or more times as efficient as, for example, an electrical heater. However, heat pumps are not suitable for providing instant hot water, such as when a user turns on a tap. Specifically, heat pumps have limits on how quickly they are able to start up and how often they are activated. It can take well over a minute for a heat-pump to switch on, perform self-checks and actually provide any hot water. This is not acceptable for a user who wants hot water for 20 seconds to wash their hands. In addition, heat-pumps typically cannot be activated more than six times per hour and can be unavailable for periods of time such as during a defrost cycle. To address these problems, the present applicants have developed a heat storage arrangement as described in GB patent application filed 7 ^th February 2021. This uses phase change materials, such as paraffin wax, to store energy in a highly space-efficient manner by utilising materials that change from a solid to liquid (and vice versa) at the relevant temperature. A heat-pump is used to heat an energy store containing the phase change material to a temperature at which the material changes from a solid to a liquid. This phase change absorbs a great deal of energy which is then available to be recovered without activating the heat-pump when a single tap is turned on. Using phase change material makes the energy store very compact, i.e. it can store a lot of energy for its size. Cold water from the mains supply is fed through the energy store to provide smaller quantities of hot water (enough, say, for a shower). Prolonged demand for hot water is met by activating the heat-pump but even in this case the energy store is used to provide hot water until the heat-pump is fully operational. One difficulty with this arrangement is loss of heat from the energy store. This typically operates at around 50º centigrade, i.e. around 30ºC higher than the ambient temperature in most homes. Demand for hot water tends to be quite intermittent with peaks of demand in the morning and evening but little demand throughout the day and night. This means that the energy store is vulnerable to loss of heat over quite long time periods. The store is, of course, insulated but there is a limit to the amount of insulation that can be included in the hot water supply system which is typically dimensioned to replace an existing gas boiler. It is an object of the present invention to ameliorate this problem. According to a first aspect of the present invention there is provided a hot water supply system located in a cabinet, the hot water supply system comprising: a thermal energy store for storing energy received from a heat-pump, the energy store including a first layer of insulation, a heat-pump send connection and a heat-pump return connection connected to the thermal energy store, a cold-water inlet coupled to the energy store and a hot water outlet coupled to the thermal energy store, a controller, heat generating components, and air flow directing means arranged to direct air from the vicinity of the heat generating components to the vicinity of the thermal energy store thereby reducing the temperature differential between the energy store and the air surrounding the energy store. The arrangement of the hot water supply system within the cabinet is intended to maintain an internal temperature within the housing, particularly around the energy store, that is higher than the ambient temperature. By reducing the temperature differential between the thermal energy store and the surrounding air (within the cabinet), the heat loss from the energy store will be reduced. The thermal energy store preferably comprises phase change material to store energy in a space-efficient manner. The internal temperature of the cabinet will be raised by heat loss from the energy store but is also raised by the heat dissipated by other heat producing components such as the power electronic device used to control the supplementary heater. Thyristors or triacs will typically also be used to start and stop the heat pump, control electrically-controlled valves and so on. These devices are preferably provided with heatsinks. The airflow directing means preferably comprise at least one baffle. One or more baffles may be located to exploit convection currents within the cabinet. Alternatively, or in addition, the airflow directing means may comprise at least one duct. According to a preferred embodiment, the airflow directing means comprises at least one fan. The fan is preferably under the control of the controller, which activates and deactivates the fan in response to temperature sensor outputs. According to another preferred embodiment, a second fan is arranged to distribute cooling air to the heat generating devices. This provides a safety feature, should the heat generating devices get too hot. The second fan may rely on cooling air is derived from the cold-water inlet and the cold water inlet may be provided with at least one vane in the path or air from the second fan. Alternatively, or in addition, the cooling air is obtained from outside the cabinet. In many embodiments the heat generating devices comprise power semiconductor devices. These device are preferably provided with heatsinks. To minimise heat loss from the energy store, the cabinet is preferably substantially airtight. The cabinet also preferably comprises a layer of insulation. In order to better manage airflow within the cabinet, it is preferably provided with an aperture towards the bottom thereof which aperture may be opened and closed under control of the controller. To further manage airflow within the cabinet, it may be provided with an aperture towards the top thereof which aperture may be opened and closed under control of the controller. In order to best manage the resource stored in the energy store the HWSS is preferably provided with at least a first electrically-controlled valve arranged to control water flow through the energy store and a second electrically-controlled valve arranged to control water flow through a supplementary heater. In order to perform this control, the controller is arranged to determine the relative flow rates of water through the energy store and the supplementary heater. The controller is preferably also arranged to activate and deactivate the heat pump. Brief description of the Figures The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic view of a HWSS to which embodiments of the invention can be applied, Figure 2 shows a perspective view of a HWSS cabinet according to an embodiment of the invention, Figure 3 shows a diagrammatic representation of a HWSS according to an embodiment of the invention, and Figure 4 shows a diagrammatic representation of another embodiment of a HWSS. Detailed description Figure 1 illustrates schematically an installation according to a first aspect of the disclosure. The installation 100 includes an in-building hot water supply system (HWSS), represented by the box 110 and a heat pump 120 (which will generally be located outside the building), which is arranged to heat water in the HWSS 110. The HWSS includes at least one outlet 130 such as a tap or shower outlet. The HWSS further comprises an energy storage arrangement (ESA) 140 which contains a mass of phase change material (PCM). A processor 150, which may also be referred to as a system controller, is arranged to provide a signal to the heat pump 120, if appropriate, based on the opening of an outlet, such as outlet 130, of the hot water system. The mass of phase change material has enough latent heat capacity to heat a predetermined quantity of water to a predetermined temperature in the interval from the opening of an outlet of the hot water supply system until at least the heat pump begins to heat water in the hot water supply system. A phase change material such as a paraffin having a phase transition temperature of 50ºC is suitable but alternatives and suitable combinations of materials will be apparent to the skilled reader. The HWSS 110 is supplied with water to be heated, for example from a cold-water supply 160 and at least one flow transducer 170 is included in the flow path between the supply of water to be heated and the at least one outlet 130. Preferably the heat pump is arranged to heat water in the HWSS by means of a closed-loop arrangement, indicated schematically by pipework 190, and the supply of water to be heated is connected directly to the HWSS as shown by pipework 200. The flow transducer 170 may be located on the supply side of the hot water system, or on the outlet side of the whole system. The HWSS is provided with at least one temperature transducer 210. If only a single temperature transducer 210 is provided, this should be in the flow path between the energy storage arrangement 140 and the at least one outlet 130. The HWSS 110 also includes an instantaneous supplementary water heater 220 in the flow path between the energy storage arrangement 140 and the at least one outlet 130. When an instantaneous water heater 220 is included in the HWSS, it is preferable to include a temperature transducer in the flow path between the energy storage arrangement and the instantaneous water heater, as well as one between the instantaneous water heater and the at least one outlet 130. The instantaneous water heater 220 is preferably an electrical heater. The processor 150 is coupled to the flow transducer 170, to memory 151, to each of the temperature transducers 210, and to the heat pump. In addition, one or more sensing arrangements in the ESA are preferably coupled to the processor, so that the processor is aware of the status of the ESA. The processor 150 is also connected to instantaneous water heater 220. With the preferred configuration in which the heat pump heats water in the HWSS by means of a closed loop arrangement, a heat exchanger, not shown, receives on one side liquid heated by the heat pump, and on another side water in the HWSS which is to be heated. Preferably, the heat exchanger forms part of the ESA. Preferably, the heat exchanger includes some or all of the mass of phase change material. As shown in Figure 1, by virtue of its size, the heat pump is typically located outside the building that houses the HWSS. The heat pump, whether it is mounted outside or within the building, will typically take 30 to 60 seconds to start to provide heat after having received a start signal. This is because an internal processor of the heat pump typically has to make checks on several components and subsystems, and also because of the inherent lag in the starting up of the compressor and the pump(s) of the heat pump, etc. Even after the start-up of the heat pump, there is of course an inevitable delay before heat from the heat pump reaches the HWSS. Likewise, some time is required for heat to be transferred, across any heat exchanger, between hot liquids supplied by the heat pump and water to be heated in the HWSS. Moreover, heat pumps are also typically configured to avoid starting more than 6 times per hour (it depends on manufacturer, but the figures are similar between manufacturers), and the processor 150 of the system will be aware of this constraint as it applies to the connected heat pump, and will also know about its own history of sending start instructions – and will factor this information into its decisions about the management of the various heat sources available to it. The processor 150 of the HWSS is configured to provide a start signal to the heat pump based upon signals received from the flow sensor 170. As noted above, the installation is so configured that a time interval occurs between providing a start signal to the heat pump and the heating of water in the hot water system by the heat pump. This means that, absent a store of hot water or some other source of hot water, the turning on of a tap or shower fed by the HWSS will involve a long wait – of well over a minute, before hot water emerges from the outlet. This is both frustrating for users and a tremendous waste of water. The wait for hot water could be shortened by using an instantaneous water heater until such time as hot water arrives from the heat pump. But with such an arrangement, given that most instances of hot water usage from a tap (as opposed to from a shower) are less than 60 to 90 seconds, most of the hot water used would come from the instantaneous water heater (either electrically or gas powered), so that the green energy benefits of providing heat pump would largely be lost. The processor 150 can modulate the power output of the electric element 220, based on a value received from the temperature sensor 210 to achieve the correct target temperature at the water outlet 130. The ESA is provided as a means to bridge the gap between demanding hot water (i.e., the opening of the tap or shower control) and delivery of hot water heated by the heat pump. The system is preferably configured so that the ESA is charged with energy from the heat pump. The processor 150, using temperature information from the temperature sensors (210) and flow information from the flow sensor(s) 170, is configured preferentially to use the ESA to heat water supplied through the outlet 130. In this way, the processor 150 minimises its use of the instantaneous water heater 220. Hence the processor is configured to provide a start signal to the heat pump based upon signals received from the flow sensor, the installation being so arranged that a time interval occurs between providing a start signal to the heat pump and the heating of water in the hot water system by the heat pump, the energy storage arrangement containing a mass of phase change material having a latent heat capacity sufficient to heat a predetermined quantity of water in the hot water system to a target temperature at least until water in the hot water system is heated by the heat pump, so that hot water can be supplied from the controllable outlet in the interval between the sending of the start signal and heating by the heat pump of water in the hot water system. The target temperature may be set based on the preferences of the users of the system, but will typically be in the region of say 40 to 45 Celsius. The predetermined quantity may be based on a desired duration of water supply at a flow rate chosen based either on the normal flow rates used for the outlet of the HWSS having the highest flow rate, or a lower rate that is considered to be both adequate and acceptable. The system and processor are preferably configured to allow these two variables (temperature and quantity) to be adjusted within preset limits (the preset limit values possibly being adjusted on the installation of the system). Information from the flow sensor(s) 170 can tell the processor 150 whether, for example, an outlet that has been opened is a shower outlet, or a wash basin outlet. If the processor 150 determines that a shower outlet has been opened, the processor 150 will send a start signal to the heat pump, because it is expected to be worthwhile firing up the heat pump for the several minutes that a shower typically takes. Conversely, if the flow rate information supplied to the processor 150 suggests that a wash basin tap has been opened, the processor will determine that no start signal will be sent to the heat pump, because the heat pump is unlikely to be able to provide hot water before the wash basin tap is turned off again. The processor 150 may be associated with logic, and for example a machine learning algorithm, that enable the processor to learn the behaviour of the occupants of the premises served by the HWSS, enabling the creation of a database from which the processor 150 can reliably predict the quantities and durations of hot water demand/usage according to time of day, day of the week, water outlet used, et cetera. Such an approach can be enhanced by, for example, providing further flow sensors associated with different water outlets of the HWSS (some or all of them, and preferably at least enabling the ready distinction between outlets with brief hot water demands – such as cloakroom hand basins, and those with prolonged hot water demands, such as showers and kitchen sinks). Additionally, by providing one or more flow sensors in the cold-water supply, it may be possible to identify cold water usage corresponding to flushing the toilet, for example, from which can be inferred the imminent demand for a brief supply of hot water for hand washing. The system processor 150 is preferably provided with logic to control all various heating assets (ESA(s), instantaneous water heater(s), and heat pump) in the most effective, economic and efficient way. It can be considered that the ESA 140, the heat exchanger, the processor 150, the instantaneous water heater 220 and the flow and temperature transducers, 170 and 210, together constitute an interface unit 250 which interfaces between heat pump 120 and an in-building hot water system 110. Figure 1 shows such an interface unit serving only to heat water for an in-building hot water system, but it will be appreciated that in many parts of the world there is a need for space heating within many buildings, and that it is attractive to use a heat pump for such space heating. Typically, current combi boilers used even in small dwellings are large enough to provide 24kW and more of hot water which is equivalent to shower or bath flow, but the typical space heating energy demand is much lower – typically around 4kW. If one were to design a system in which both domestic hot water (DHW) and space heating were provided solely from a heat pump, one would need to specify a 24kW heat pump to meet the DHW requirement, but such a system would be impractically large for typical 1-to-3 bedroom flats and houses, where for the most part the need is for space heating with only intermittent hot water usage. Figure 2 shows a perspective view of a HWSS 200 according to an embodiment of the invention. The HWSS is contained within a cabinet 202. The cabinet has plumbing connections on the side for the flow 204 and return 206 pipes of a heat pump (not shown). Heat from the heat-pump is used to heat water from a cold main pipe 208 to provide a hot water output 210. The cabinet 202 is entirely air- tight except for a small aperture 212 in one corner of its base. The aperture may be required since the air temperature (and hence air volume) within the cabinet can vary but the aperture is designed to be as small as practical to minimize the loss of heat from within the cabinet. While the Figure shows the location of various plumbing fittings, the skilled reader will appreciate that these may be located at other points on the cabinet. While the small aperture is located on the base of the cabinet, it may be located on one of the other panels, or even omitted, provided that when it is present, it is located towards the bottom of the cabinet. Figure 3 shows a front diagrammatic view of a HWSS 300 located within a six-sided rectangular cabinet 302 whose front side panel has been removed to illustrate the embodiment. The cabinet has a layer of insulation 304 and is airtight apart from a small aperture 312 located in a bottom panel. A heat or energy store arrangement (ESA) 306 has a send coupling 314 receiving heated fluid from a heat-pump and a return coupling 316 for returning the fluid to the heat pump (not shown). The ESA is heated in an indirect sense by a heat exchanger within the ESA. The HWSS has a cold-water inlet 308 from a tank or mains water and a hot-water outlet 310. Heat from the ESA can be transferred to water from the cold-water inlet using another heat exchanger within the ESA. The cold-water inlet is connected to a temperature sensor 318 which provides a temperature value to a controller 320. The controller is also informed about the degree of energy storage in the ESA and can determine whether the ESA can heat the water from the cold-water inlet sufficiently. If the answer is no, then the controller can activate a supplementary heater 322 via a power semiconductor device 324 such as a triac or thryristor. The device 324 preferably has a heatsink to transfer heat dissipated in the device to the surrounding air. The supplementary heater is an electrical heater arranged between an output of the ESA and the hot-water outlet of the HWSS. The power semiconductor device 324 is located beneath the ESA 306. Any heat dissipated by the semiconductor device will warm the surrounding air which will be drawn upwards by convection. The warm air is thus drawn to surround the ESA, reducing the temperature differential between the ESA and the surrounding air reducing heat losses therefrom. One or more baffles or ducts could be arranged to direct the warmed air. Figure 4 shows another embodiment of a HWSS 400 according to the present invention. A cabinet 402 includes send 414 and return 416 paths for a heat-pump (not shown) as well as a cold-water inlet 408 and a hot water outlet 410. The send and return from the heat-pump are connected to an ESA 406 which is provided with an insulating layer or jacket 404. Energy is provided by the heat pump and stored in the ESA and the stored energy can be transferred to water from a cold-water inlet 408 as before. The cabinet includes a layer of insulation 456. The arrangement of the embodiment in Figure 4 differs from that of Figure 3, inter alia, by providing two distinct water paths through the HWSS. A first path proceeds from the cold-water inlet 408 to the ESA via a temperature sensor 432 and a valve 418 and a second path proceeds from the cold- water inlet to a supplementary heater 420 via a valve 422. A controller 430 determines, using temperature information from sensor 432, the extent to which the supplementary heater is to be activated and the relative flow of water through the ESA and the supplementary heater. The controller comprises a plurality of inputs from sensors such as sensor 432 and a plurality of outputs 460 to drive power electronic devices discussed further below. The heater 420 and the valves are controlled by respective power electronic devices 424 arranged adjacent a pair of baffles 426, 428 which direct air (arrows H) from the power electronic devices upwards towards the ESA 406. Baffle 426 may be omitted and the side of the cabinet used to direct the airflow although this may dissipate heat via the cabinet wall. Front and back baffles may also be provided (not shown). One or more ducts could be used alternatively or in addition to one or more baffles. The power electronic devices are all preferably provided with heatsinks. A temperature sensor 438 determines the temperature of the power electronics and reports this to the controller 430. Other heat generating components of the system may be arranged in a similar position to the power electronic devices. While convection may be used to transfer air within the cabinet, it is preferred that a heating fan 434 is provided adjacent the power electronic devices to direct air from within the cabinet over the devices and between the baffles 426, 428. The heating fan itself will be driven by one of the power electronic devices 424 under control of the controller 430. The fan 434 may be activated when the controller 430 determines that there is spare heat in the devices 424 that could usefully be redistributed to the vicinity of the ESA 406 (for example, by reference to the temperature sensors 438, 452). A further fan, a cooling fan 436, may optionally be provided. Although most power electronic devices are capable of operating at 80ºC and are unlikely to overheat, if the controller determines that the devices are too hot then the fan 436 may be activated. Again, the fan will have its own power electronic device among the devices 424. The fan 436 has two possible sources of cooler air for cooling the semiconductor devices. Firstly, an aperture 440 in the bottom wall of the cabinet may be opened using an actuator 442. This allows ambient air to be drawn into the cabinet by the fan 436. To ensure that sufficient air enters the cabinet, an aperture 444 at the top of the cabinet may be opened using an actuator 446. Secondly, the fan may be arranged to blow air over the cold-water inlet 408. The pipe will typically be cooler than ambient air temperature due to the flow of cold mains water. The pipe may be provided with vanes 448 to allow the cold pipe to extract more heat from the air driven by fan 436. The controller 430 is provided with inputs from various sensors to permit appropriate decisions regarding the air management within the cabinet. A sensor 432 detects the temperature of the incoming cold water and a sensor 450 detects the temperature of the outgoing hot water. The controller uses these values to determine whether or not to activate the supplementary heater 420 and the relative flow rates through the energy store and the supplementary heater. The controller is also provided with the temperature of the air within the cabinet (preferably adjacent the ESA) by a sensor 452 and also the temperature of the power semiconductor devices by a sensor 438. Outputs from the controller are connected (not shown) to the power semiconductor devices 424 in order to operate the valves, heater and fans. The controller may be provided with external information such as weather forecasts and may also be programmed to log the behaviour of air within the cabinet in response to various actions. These actions may include amount of hot water drawn, activation of the fans 434 and 436, opening of the apertures 440 and 444 and so on. This log may then be used by the controller in future situations to determine whether to activate a fan, for how long and at what speed. The controller is also arranged to provide start and stop signals to the heat pump (not shown). The controller may be arranged to start the heat pump when it is determined that the energy store is too depleted to meet the demand for hot water. This may occur when the water flow through sensor 454 indicates that the user is having a shower or running a bath.