Had another go at “Dead Zone”, Zone 7, and found and collected the Emerald again, then went on to complete the zone and defeat the boss at the end – who turned out to be Blaze the Cat, who was easy to beat.
Then “Final Zone” opened up on the map, and it’s a boss fight with Eggman Nega (yes, it wasn’t Eggman after all) in a giant mech suit. I tried a few times to kill him, but failed, as I couldn’t see how I could make it up his arm to jup on his face after a few hits, as he kept shaking me off.
Figured it out now, so should do better next time!