I got there fine and killed everyone and the leader easily, so I went back to hand in the quest...
Only problem is it's impossible.
As soon as I enter the Companions building, the quest is moved from my "In Progress" section into the "Completed" section, but the quest still tells me to hand it in to Farkas. I talk to him and have no option of handing it in. When I ask someone else for a mission they just tell me "I'm already on a job from Farkas".
If I leave the building the marker points me back into it, but then when I enter it shows the "Enter door" symbol miles away in a place that's impossible to reach.
Anyone know how to fix it? It's the first gamebreaking bug I've encountered.