I had trouble with this quest also.
First time I tried it, I was too low level, so I could be seen by all guards.
So I went back when I was 19, and snuck around the place, went and rung the top bell, then the main floor bell, when two red guards popped out of nowhere.
So I done them in a different order, the two guards appeared again.
So I tried again, in yet a different order, and managed to complete it.
So yeah. I think it might be bugged, or ya just have to ring the bells in a certain order..
Dunno.