So you came along the main road, so would have passed the bus lane sign, and would seem to have driven in it before you got to the zig-zags, true ?
Bus lane PCNs can be overturned in many cases due to a quirk of the legislation under which they are issued, but you need to take the council to London Tribunals, and risk the full PCN penalty if you lose. Hippocrates on here can explain it to you and hopefully will soon.