Had a similar problem last week. I found a guy on eBay that when given the serial number AND model number of the radio, emailed me the correct code for the princely sum of 99p
I didn't know the model number of the radio (mine is a 6 CD changer in a K12 and had no sticker on the back of it, but the serial number engraved on the side) but Googled for pictures of Micra radios and found a couple of pictures of identical units that showed details of the model number on the rear views.