I have a gas stove top which is about 15-20 years old. The round plate things that surround the flame (there's probably a name but I cant think of it right now!) are stained. I have scrubbed them with Jiff, ajax, spray and wipe, detergent.....but they still wont clean.

Am I doomed to grotty stovetop plate thingos forever? Or, can someone tell me how to clean them???!!!!!

Much thank yous.