I wouldn't have thought you would have to remove the bumper. I've just ordered my copy of the new haynes manual so I'll be in there so I'll post up the answer when I get my little book if no-one can tell you before I find out.
don't know if the morrette headlight conversion is the same as the standard headlights but all I did was unscrew the two screws on the top of the light and then un bolt the bracket on the inside of the light at the bottom and then they just pull out with a bit of gentle persuasion (if thats how you spell it) :wink:
Yes you do have to remove the bumper on the Corsa C. Its not difficault. There are 12 screws and fixings total around it, (under the front, under the wheelarches, and on the slam panel) and it simply lifts off, its not heavy. Then you can get at the third headlight fixing screw underneath it. If you force the bumber out of the way you risk cracking it, its only thin plastic.