I can't see it will ever be finally resolved, unfortunately.
Although the local problems were made much worse due to the move of the Sylvester Road delivery office to Peckham, Royal Mail has had issues ever since it was privatised.
Whenever the local MP gets involved, things get a bit better temporarily, then they just deteriorate again.
I hope I'm wrong, but there are similar postal delivery issues all over the country.
I think there are currently discussions ongoing to change the delivery requirements which Royal Mail is supposed to adhere to for first and second class post.
Silly question, but are you actually expecting letters? I haven't had any post for ages either, but I'm not expecting any! Much stuff these days is done by email or on apps.
The East Dulwich delivery office is in Peckham, not West Dulwich?