I'm somewhere between the two on this. I don't think one can definitively say the charge was 'clearly' cancelled, as the only reference to it being cancelled is in a letter that also says that the appeal has been rejected, and that payment or a POPLA appeal are now due. Contractual disputes don't work as some sort of game where whoever makes an error first automatically loses. However, the parking company's error in this case has materially disadvantaged the OP, by at best creating much ambiguity, and at worst leading the appellant to believe the matter was closed, such that they declined to submit a POPLA appeal.
With that in mind I agree with b789's approach of going in hard on the complaints route.
If it were to get to court, I think additional arguments than just "they said it was cancelled" might be needed, but, more likely, it would be discontinued anyway, this being Euro Car Parks and DCB Legal after all...