You only have to report criminal offences, including those dealt with by a fixed penalty notice (such as speeding).
Driving in a bus lane is entirely a civil matter, which isn't (and, indeed, can't be) dealt with by a fixed penalty notice.
A penalty charge notice, which is what you received, is simply a bill from the local authority for using a bit of their road that you weren't entitled to drive on. Just as you don't need to tell your insurer about a bill you get from a garage, you don't need to tell them about a bill from the council either!