Sorry, but I have to ask why, when your informal challenge to a PCN served, (presumably), has been rejected, you have written back to them. They are under no legal obligation to respond to your second informal submission.
The enforcement process is that if you want to take the matter further, you must wait for the Notice to Owner, and submit a formal representation against this. This begs the question - are you the owner, and is the address on your V5C Registration Certificate up-to-date.
Assuming you are the owner, at the NtO stage the discount option is lost, although many councils do re-offer it when rejecting formal representations. If representations against an NtO are rejected, and no re-offer of the discount is made, then it becomes a complete no-brainer, to take them to the Traffic Penalty Tribunal, because the penalty remains the same, and there are no additional costs.
Therefore to help us, please post up the PCN, all sides, and your original representations. Redact nothing except name and address on all documents you post. We don't often see PCNs from outside London, and the "small print" on them can sometimes be so wrong as to make it void. PCN content is mandated in regulations, hence the need to see all of it.