You'll need to get a neutral from somewhere - if there isn't one in the switch it will have to be from another light - take a live and neutral from the original light to the new one, live to the terminal marked 'loop' or connector block together with the brown core of the cable you have just put in, neutral to the neutral of the light fitting, and the blue wire of the cable you have just put in to the live of the light fitting, with a brown sleeve around it.
At the switch connect the brown wire to COM, and the blue wire (again with a brown sleeve around it) to L1.
Adam
February 2010