All you can do is wait for their reply.
IMO, if the registered keeper(to whom any Notice to Owner would be sent) wants to continue then I suggest they say what to me is obvious i.e. why has the council chosen to place DYL at this location, which might encourage a BB holder to park when, according to them, there is a more onerous and unsigned prohibition in place against which a BB does not provide an exemption. The driver was aware of the Highway Code's references, but these are incomplete because they do not refer to (possibly because regulators do not anticipate) DYL and dropped footways being placed together.
It might make them think, if nothing else!