So I presume you drove slowly along Ferndale Road towards the main road looking for a parking space, and saw a car preparing to leave. You therefore stopped in front, and waited for the car to leave the bay, then you backed-in.
For this kind of thing it's imperative we see the TfL video of the alleged contravention. You can obtain this by contacting TfL and requesting it. TfL,being in the Dark Ages, send these out as DVDs and also place the PCN on hold for two weeks. Within this time it is expected that the DVD will have been produced and arrived at your address. So request the video, and post it up here when you get it. Do be aware that the "On Hold" automatically ceases after a fortnight, so if the DVD is not received after about 10-11 days contact TfL again to find out when it was sent, and to request a new On Hold period.