Who actually sent the assassin to kill Brandon Stark? Jamie believes it was not Cercei, and swears he did not do it - and it makes no sense for him to lie to Catelyn about that since he admitted to her that he pushed him out of the window.
Great question. I am simply providing an answer so this question is no longer listed as Unanswered.
As stated, this question was already answered here on SciFi (as System Down mentioned in his comment)