Light the grill and after a few seconds look under it; are both sides of the central burner strip alight and are both sides of the metal mesh glowing red hot? Sometimes only one side of the central strip is burning (and therefore the other side is emitting unburnt gas). Don't look too closely if there is unburnt gas in the grill compartment that suddenly catches alight; don't ask me how I know.
I think this is a known fault, with dissimilar metals, a lot of heat, moisture from boiling over etc. I had to drill mine out and replace with a self-tapping screw. Unfortunately, penetrating oil applied at the top doesn't get down to where the grub screw goes into the base plate.
Second Norfolk Marine. Our cooker, only a couple of years old started smoky flame on one burner. I contacted NM and they sent me a new jet for each ring, I had purchased the cooker from them new and they were FOC. The burner rings can get a bit dirty and regular brushing helps, but in our case it was the jet at fault. Worth taking the rings off regularly to stop the screw seizing up.