You will need to take a live and neutral from another light on the same circuit (your lounge light by the sound of it) to the new light in the porch.
Obviously this will mean the light would be on all the time, so you need to take a 2 core and earth from the light (brown wire) to the switch and back again (blue wire with a brown band or bit of tape to indicate it can become live).
Adam
October 2009
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