Looking at your GSV link, there is a wide pavement with paving slabs. I assume you parked partially off the carriageway on that pavement; correct ? If so, then you would appear to be bang-to-rights as parking off carriageway in London has been an offence since the 70s.
Sorry if this isn't what you wanted to hear, but it is reality, I'm afraid. Having said that, there are other things that can get PCNs cancelled, and our adminstrator, CP8759 is very skilled at finding them out. The only downside is that one normally has to take the council (or TfL), to London Tribunals as representations to councils/TfL are usually rejected in about 95% of cases.
So see what the others say, but don't miss any deadlines.