Reason for suspension was "domestic" so hardly an emergency, so one would expect the sign to have been put up more than 3 days in advance. When did you become aware of the PCN on your car ?
Their photo shows the suspension sign was very close to your car, so difficult to see how you would have missed it if it was there on the 3rd.
I suggest you submit reps stating that when you parked legally on 3rd March you saw no suspension sign and would have expected more notice for what is a routine suspension, ("domestic"). They should reply to say when the sign was put up.
