I found that most cleaning products around do next to nothing on them, they can get rid of some of the surface marks but the burnt in stuff stays forever.
You can get something called Collo Electrol, its application makes it seem a bit like shoe polish for hobs. It basically brings them back to black, you apply a thin layer onto a luke warm solid plate, then turn it on and let it burn in. You'll need to close any doors to that room and open the windows as it can create quite a bit of a smell and smoke somewhat but the end result is that its back to looking like a factory finish almost.
Picked up mine from UK Whitegoods
Appliance Spare Parts Solid Hob Plate Cleaner & Preserver For Electric Hobs & Cookers