If you're going to make a complaint, do everything you can straight away. Request CCTV from the bus company and the council (street CCTV), give the witness details, complain to TfL, the whole works. Don't just email TfL and wait, because you'll get an email in 3-4 months saying that they can't do anything, or asking for some small piece of information, which they'll then go away and think about for another 3-4 months...
If you're going to make a complaint, do everything you can straight away. Request CCTV from the bus company and the council (street CCTV), give the witness details, complain to TfL, the whole works. Don't just email TfL and wait, because you'll get an email in 3-4 months saying that they can't do anything, or asking for some small piece of information, which they'll then go away and think about for another 3-4 months...