Just go in the 9 adit, up the incline, and just before the collapse, find your way to the right and up a backfilled chamber, and you'll pop out in the bottom of the West Twll. Have a poke about, heading towards Croesor, if you don't know where you're going, you'll find plenty to explore.
Finally either return out of 9, or climb the Twll to the top.
Chris.