I really haven’t noticed any signage…
Not one, not two, but
three NRT signs, so claiming insufficient signage doesn't look a credible appeal argument.
However, the good news is that TfL PCNs can be easy to beat, based on 'technical' appeal arguments developed by our admin and also a couple of contributors with familiarity with London so wait for their input
However, your immediate first step, (if you haven't done so already), is to phone TfL and ask for the video, which puts a fortnight into the enforcement timetable, so gets you, (and more importantly, us), off the hook timewise. They will send you a DVD. and freeze enforcement for 2 weeks. TfL seem strangely reluctant to put their videos on-line like all the London councils, one has to wonder why.