I'm answering from Chicago, so I hope we share a common language! If by hob ring, you mean the metal "dish" surrounding the burners on top of the range (or stove), I would try soaking in soap, white vinegar, and boiling water, then scrubbing what may have come softened and loose. A sturdy wire brush may help. Multiple soakings may be in order.

If there are decades of burned on, carbonized food, I'd either determine the level of "clean" I'd be willing to live with, or contact the manufacturer for replacement parts.

I have just moved house and the new oven has two metal hob rings that are absolutely filthy! I have spent the last two days trying to scrub them with a scourer to clean them up but it isnt working. I have used antibacterial spray and wipes and also this oven cleaner stuff....

Does anyone know how I can get rid of this filth? It looks like its been on there a long time!