You committed the offence when you failed to provide the information within the original 28 days.
There is no provision within the statute for the police to extend the 28 days, but they often allow you a second chance and would not prosecute if you complied with that, as it would not be cricket to do so.
The requirement is for the information to be served on them (delivered). The second chance was for the information to be served on them within the 14 days. Unless you are intending to argue that the notice that you have already told us you posted after the expiration of those 14 days was served on them within the 14 days, and that they are lying about not receiving it, I'm struggling to see what your defence is.
