The 'lense' is a whole lamp unit. The only problem in fully undoing the clamp is that a part will fall off and have to be found and part screwed back B 4 you refit the new unit. Are you pressing down too hard on the screwdriver handle? You may have stripped a thread in plastic. Try turning with the lightest possible downward force possible. Tip for young players: When refitting a self tapping screw into plastic part turn the screw backwards first to feel where the existing thread starts. Try not to start a new thread as you will run out of plastic as the hole increases in size as plastic is cut away by the screw.
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i