Let's hope the assessor can follow, assuming it gets that far.
OP, on points of accuracy:
The incorrect wording could mislead the keeper into thinking they have one less day to name the driver or challenge the charge before the operator claims the right to pursue them.
No. The keeper can relieve themselves of liability by naming the driver at any stage before legal proceedings are commenced anyway. And no, appealing has nothing to do with the wording of 9(2)(f), in fact PoFA is virtually silent on matters of appeals. Here's what it says:
9(2)(g)inform the keeper of any discount offered for prompt payment and the arrangements for the resolution of disputes or complaints that are available;
The point of 9(2)(f) is not just that the creditor may not hold the keeper liable before the prescribed period has elapsed, it's that their Notice to Keeper must inform the keeper by stating this clearly.
Personally, your reference regarding what the incorrect wording could lead to is unnecessary and I'd leave it out.
So far we've skirted the issue of whether there was a breach and if so its particulars. Even Horizon's rejection was silent on this point.
I would add to your appeal that you haven't seen any evidence or reasoning which specifies the breach and therefore have been unable to consider this and make reference in your appeal.
Do you know the substance of the breach? Being on site for the period specified isn't a breach as such. It might be if only 10 minutes was allowed, but not if 3 hours was allowed!