murphwiz is right.Just spent half an hour taking enough of the bumper off to see the retaining clips.
The light unit has to be pulled/pushed forwards (the direction the headlights shine) by about an inch to release the spring clips.
Engineering motto " if it needs excessive force you are doing it wrong"
Try easing it forward from behind with a lever/wrecking bar well padded with cloth to protect the unit and surroundings, or inflating a car inner tube in the gap behind it. Less likely to cause damage than yanking it forward, especially the first time before lubrication applied.