I only found this by accident, it's a more or less impossible puzzle. In the toilet/lava pit area there are lava flows falling down both sides of one of the walls. These aren't actually lava, they're water cunningly disguised as lava. In one of them there is a lockbox that opens the un-frobable iron door in one of the corridors next to the lava room. After then it's pretty straight-forward.
Z