The weapon was a switch-with-things hook hoodilly. So, not really a weapon then, actually. Although you do use it on the end boss in that dungeon, as expected.
With that dungeon out of the way, I was sent on a trek through time and, er, place to find bomb flowers to help a Goron elder. Of course, that required a sub-quest, which involved throwing bombs as the Great Moblin who muttered something about me defeating him in Holodrum (which I never did). Odd.
Anyway, I was then able to get into Dungeon 5, which has lots of red and blue blocks that I need to swap between a lot to reach various areas. The mid-boss was very easy indeed, and required me to race him to a ball, and chuck it at him before he chucked it at me. I’ve found the map and compass, and also the weapon here – the Cane of Somaria. It was entirely expected that it’d be the weapon I got though, as some of the puzzles were crying out for it. I just have the Big Key and end boss to find and do now.