ThePaleHorsemans guide was spot on, brilliant find!
This is just a video showing where you have to delete your game and what happens.
Skip to 3:00 to see me exiting the cave and where i actually go ahead and delete the save, and as i go back into the game to unlock it!
The first 3 minutes is me explaining it and sort of getting through the last main level just so you can see where it sort of starts off.
Hope this helps