Went back to Zone 2 and picked up the Chaos Emerald there, before trying Zone 6 again. Managed to get through it this time, and beat the boss.
Then it was on to Zone 7, and shock! It’s set on a spaceship, and there are gravity flippy things! How completely new for a Sonic game!
Found a Special Stage on Act 1, and failed to complete it because it’s too hard. Luckily, there was the right sort of level structure around the stage entrance to boost my Trick Bar up enough to have another go. And another. And another. $hlmun tries later, and I finally managed to get the final Chaos Emerald!
Then I died on the level and it was Game Over, so I lost it again. RUBBISH.