YOur last post was over a month ago, so please bring us up to speed by posting their letter of rejection of your informal challenge.
The council have up to 6 months to serve a Notice to Owner, so it's early days yet. Some councils can be very dozy. The NtO is sent to the address on the V5 Registration Certificate for the vehicle. So is it your car, and is the address up-to-date, and the same as it was at the time the PN was served ?