The zone entrance signs are on the boundary of the zone, the car was in the Croydon Central zone, see the map
here.
Like many zones, it covers multiple streets so to defend the allegation on the signage, you'd have to determine exactly where the car entered the zone, and then you'd have to go and check the signage at that location. It does happen that the signs are obscured, missing, bent, turned around and so on.
On the other hand we only have until Wednesday to make reps, so we might have to limit ourselves to a plea for discretion + the 0845 point, and leave any issues of signage for the tribunal stage.