2003 td4 battery light?

Hello my 'landy chums'.. I own a range rover p38 and have just nipped across to your forum to ask for your guys help.. my mrs's 2003 td4 kal .. the battery light has started to flash..before i get me tool box out (so to speak) can any one say wether its a battery fault for deffo?? or alternator?? the battery has this green light thing on it to check for voltage (i think).. any way its green.. so any help would be great... thanks stuart.. typical my p38 is allways the sickly one... the freelander's never missed a beat!!..