Thanks fungus! Cleared up a few things.
I ran a formula and worked out i should use a 2.5k ohm resistor for the TIP120 running a 1W LED. Does this sound in the ballpark?
Bit wary of running a led straight off the power supply - so i saw some cheap 1w led drivers on ebay. They run on 12v. Do you think i could switch one of these with another resistor and an external power supply?
Thanks for the help