The left hand bulb went on the girlfriends Eos, was the bulb and costs £140+ from VW
if the bulb has a metal box attached to it then the bulb is a 'D1S'. Can pick them up on ebay for £30-£70, best to get a pair as they colour changes slightly over time and the one you have left will probably be different.
Not sure if changing the bulb is that much different on an Eos from a GTI but it was a bumper off and headlights out job!
hope that helps