You should submit an informal challenge stating that the offence did not occur because you saw and inspected the suspension boards but there were no dates or times on them. Attach your photos as proof. Also, if the parking sign for the bay had nothing on it then this confirms your representations.
Please give us a GSV link to the location, although this looks to be the location: -
https://maps.app.goo.gl/mxvFLtqLNiFbK5yd6Don't be surprised if they reject your reps, because almost all councils just send Fob-Off letters in response to informal reps, as they know most people just cough up then, like about 95% of them.