Although the geographical extent of this post is the area of the parking place, this isn't parking suspension as such, it's a TTO-imposed variation to pre-existing permissions/restrictions.
OP, what's been asked for would at least allow us to see the exact wording of the notice. If there isn't one in evidence then the council have no case, the sign could just recite a nursery rhyme for all an adjudicator would know.
You have no argument regarding visibility IMO because it was your obligation to check the existing traffic signs anyway...which leads to the issue of prior notice. Presumably you are a permit holder, hence their comment about advance notice. Can you elaborate on this e.g. are you a permit holder, are you a resident in the street or do you live elsewhere, how often do you use parking places in this area etc?