1 Retrofit LED lighting system FIELD OF THE INVENTION [0001] The present invention relates to lighting systems, in particular an LED lighting system that can be used to replace halogen lamps. BACKGROUND TO THE INVENTION [0002] LED lighting has become a popular choice for energy efficient lighting systems. A 5 Watt LED light may replace a 35 Watt halogen light and produce a comparable light output. LED lights have advanced to the stage where they are of a comparable size and shape to standard halogen globes such as an MR16. [0003] The cost of LED lights has fallen to a level that is enticing people to replace existing halogen down lights with LED equivalents, however the installation costs of such are often prohibitive as the services of a qualified electrician are usually required. [0004] To reduce installation costs LED lights have been developed as plug in replacements for halogen globes, allowing a home handyman to replace a halogen lights with LED lights. In such cases the LED light is adapted to plug into a halogen globe socket and fitted with circuitry to allow it to run from the output of the transformer that would normally power the halogen globe, i.e. nominally 12 Vac. [0005] Such arrangements often do not work as the LED light does not present sufficient load for the transformer. Similarly when the light is in a dimmer circuit it may not present sufficient load for the dimmer. To overcome these minimum load requirements LED lights have been developed to draw extra power in order for the transformer or dimmer to function. Such a solution is less than optimal due to the extra power needing to be dissipated within the light which adversely affects the lifespan of the electronic components involved. [0006] The object of this invention is to provide a retrofit LED lighting system that alleviates the above problems, or at least provides the public with a useful alternative. SUMMARY OF THE INVENTION [0007] In a first aspect the invention provides a retrofit LED lighting system comprising an LED lamp and controller for driving the LED lamp wherein the controller is located in an enclosure physically separated from the LED lamp and includes an electronic load circuit. 100011 Preferably the controller includes a power input connector compatible with a power connector for use with a halogen lamp. [0002] Preferably the input power connector includes pins in accordance with the power pins of an MR16 halogen lamp. [0003] This paragraph intentionally blank f0004] It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS [00051 Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows. 100061 Figure 1 shows a prior art halogen or LED lighting system with a lamp unplugged. [00071 Figure 2 shows a prior art halogen or LED lighting system with a lamp plugged in. [0008] Figure 3 shows a retrofit LED lighting system of the invention seen from a first view. [0009] Figure 4 shows the retrofit LED lighting system of Figure 3 seen from a second view showing input pins. [00101 Figure 5 shows a lighting system that has been updated with the retrofit LED lighting system of the invention seen from a first view. [0011] Figure 6 shows lighting system of Figure 5 seen from a second showing 3 mating components. [0019] Figure 7 shows a circuit diagram of the retrofit LED lighting system of the current invention. LIST OF COMPONENTS [0020] The drawings contain the following components [0021] 10 prior art system 12 transformer 14 mains lead 16 low voltage lead 18 base 19 base socket 20 prior art lamp 22 lamp pin 30 retrofit LED lighting system 40 controller 42 mounting tab 44 base receptacle 46 input pins 50 output lead 60 LED lamp 70 updated lighting system DETAILED DESCRIPTION OF THE INVENTION [0022] The following detailed description of the invention refers to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration. [0023] The invention provides a retrofit LED lighting system that can be plugged into an existing transformer that is used for halogen lighting. Typically the transformer would sit in a ceiling space and be connected to a halogen lamp in a light fitting such 4 as a down light. For sake of clarity the accompanying figures do not show a light fitting. [0024] A prior art system 10 is shown in figures 1 and 2. The system comprises a transformer 12 having a mains lead 14 to supply incoming power, typically 240 Vac, and a low voltage lead 16 to provide power to a lamp, typically 12 Vac. The low voltage lead terminates in a base 16 including sockets 19 for accepting pins 22 of a prior art lamp 20. Figure 1 shows a lamp unplugged and figure 22 show a lamp plugged in. The lamp shown is of a MR16 variety and the base GU5.3. The lamp may be a halogen lamp or an LED lamp. In the case of an LED lamp, the lamp enclosure would typically include a driver circuit to convert 12Vac to a dc current source and optionally a load circuit to ensure that the transformer, and possibly a dimmer circuit connected to the mains input, has an adequate minimum load. [0025] Figures 3 and 4 show a retrofit LED lighting system 30 of the present invention from 2 different to enable the salient features to be appreciated. A schematic of the system is shown in Figure 7. The system provides a means of physically separating the LEDs of a lamp from LED driver and minimum load circuitry, thus allowing the separate element to be optimised from a thermal perspective. The system 30 comprises a controller 40 connected to an LED lamp 60 via an output lead 50. [0026] The LED lamp 60 physically resembles an MR16 halogen lamp so that it may be used in place of such a lamp in a light fitting such as a ceiling down light. In terms of electronics, the lamp only includes LEDs. This keeps the power dissipated by the LEDs away from other circuit components. Additionally, the thermal design of the lamp need only cope with the power dissipated by the LEDs. This is advantageous as the lamp is physically limited in resembling a conventional MR16 lamp. To help dissipate heat the body of the lamp may include cooling fins (not shown in the figures). The schematic of figure 7 shows four individual LEDs, however any other number of LEDs could just as readily be used. [0027] The controller 40 comprises a housing and internal electronics as seen in figure 7. The first end of the housing includes a base receptacle 44 and input pins 46 which together accept a GU5.3 lamp base. The output lead 50 emerges from the second end of the housing. The controller further includes mounting tabs 42 to allow the controller to be secured. The housing is elongate so as to present a small cross section, thus allowing it to pass through a light fitting to which the system 30 is being 5 fitted. The housing may also include fins to help dissipate power if needed (not shown in the figures). [0028] Figure 7 shows an example circuit for the controller 40. The actual circuit used is not critical to the invention and many other configurations could be readily conceived by any one skilled in the art. The input pins 46 accept a nominal 12Vac which is rectified to provide a dc supply. Q1, Q4, R1, R2 and R3 sink a load of approximately 0.5A to satisfy the upstream load requirements for the transformer 12 and dimmer (if present). Similarly Q2, Q3, R4, R5 and R6 sink a load of approximately 0.7A suitable for driving a string of LEDs. [0029] By having the controller circuit separate from the lamp, the power dissipated by the controller is of no significance to the lamp and vice versa. This allows the controller to include an inefficient linear current source topology for driving the LEDs. This allows the controller to be simpler and of lower cost than if thermal efficiency was a prime concern. [0030] The output lead 50 serves to connect the controller 40 to the lamp 60. The lead comprises a two core cable, but could readily be replaced by two wires instead. The lead is fixedly attached at either end to minimise cost and complexity. [0031] Figures 5 and 6 show an updated lighting system 70 in which the invention is connected to an existing transformer 12 in place of a conventional lamp. The base 18, which would normally be connected to a lamp, is instead plugged into the controller 40 which in turn drives the led lamp 60 via output lead 50. [0032] The reader will now appreciate the present invention which provides a retrofit LED lighting system that is readily installed without the need for a qualified electrician and alleviates the thermal problems inherent in prior art devices in which LED lamps include electronic circuitry. [0033] The present invention has been shown with connectors to mimic an MR16 lamp, but could readily be applied to other connector types. [0034] Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is 6 recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus. Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in this field. [0035] In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers.