We haven't seen the OP's challenge, but we can infer certain matters from the council's reply:
1. The OP claimed that they had permission to park at the location, presumably from the occupier of the premises served by the DK.
OP, if so then all you need to do is produce this to the council and you won't need assistance because the PCN will be cancelled. You might want to post here first. There will not be any need to rely on another exception such as assisted alighting.
2. On which point, the response says that the council were alerted by information from a resident and on this basis they despatched a CEO. This would appear to directly challenge the claim that the occupier had given permission. It must surely also mean that the car must have been parked for some time i.e. the occupier notices, phones the council who determine that a CEO should be sent etc. surely this must have taken at least 15 minutes!
OP, can you elaborate pl.