Help -I can't seem to get my gas hob burners really clean

[Fizzylemonade]

I have a green enamel gas hob (it was inherited with the house and it will be changed soon but not soon enough). The silver metal bits below the black discs are really discoloured.

I think the previous people have used a harsh chemical cleaner on them as they seem different to any gas hob I have owned before, but they are still stained brown/black. The rest of my hob is sparkling, I feel they are letting the side down.

Any top tips?

Have a go soaking them in a strong solution of soda crystals. Works well for me when I can be bothered.

Thanks, will locate soda crystals.