Although the delay is lengthy, there is nothing unusual in a NtO being served before any response is received relating to an earlier informal challenge against the paper PCN.
If you look on the PCN, normally on the back, there will be something on the lines of "if a challenge is submitted against this PCN it will be considered, but if the council serve a Notice to Owner anyway, the owner must respond to the Notice to Owner". So that is what you must do or it's game over. Submit your original challenge, plus the info they asked for and point out you have received no response to your original submissions.
You could also make the point that the NtO has been served over three months sinced the alleged contravention date and is an unfair delay, which you suggest is grounds on its own for cancellation. In fact they have six months to serve a NtO, but adjudicators at London Tribunals have ruled in the past that over three months is unfair as memory fades with time, thus prejudicing your recall of events if the matter is taken to adjudication.
