Hi Ian, Its rare to find a LED than will work off 6v most are much lower and you will need to add the appropriate resistor. Your LED supplier (Maplin is a good source) will have the specs for the LED's they sell.
This site has a online calculator to save you the headache
http://metku.net/?sect=view&n=1&path...calc/index_eng . You can get flashing LEDs as well.