The 'letter' is an Order for Recovery and this is issued by the council under the authority of TEC. In your case it's the last time that the council will be judge in its own cause.
Although there are strict time limits on you responding to an OfR, you responding to a NOR and you responding to a NTO there is none which applies to the council issuing a CC or OfR. C'est la vie.
You just have to wait but proactively by checking with TEC every week to 10 dayswhich it seems you've started. Alternatively, as the cost of registering the charge is £10, you'll know when it's been registered because your online debt would have increased by a tenner. If either happens, come back here for advice. The next steps are pretty standard and simple, but they are timed and might include you having to have your signature witnessed(can't be precise because we don't know the contravention and therefore the governing legislation).