I recently did mine. Just buy some cutting compound (most chandlers will sell it) to remove the grime and old polish. Then re-apply the polish to make them bright, clean and shiny
Before flailing away with cutting compund, you may find that limescale descaler from tesco's etc. The fender surface is soft, and will pick up muck more quickly once abraded. Descaler, then the cutting compund, I'd say.
Be careful with descaler. Try it on a small area first. It can leave the surface of plastic sticky and add to your problems. Try Cif cream(ex Jif) with bleach.