I would say your lights have been wired "unconventionally".
Unless there is another switch somewhere which controls the same light (the one which isn't working) I don't see what the other black wire is for.
Try:
Red (the one with the sticker) to COM, with the other red in L1 (this seems to work).
Link between two COM terminals.
Black wires in L1 and L2 in the other switch.
If that doesn't work, open up the ceiling rose and decribe what wires you have there.
Adam
January 2010