It must be something to do with the Powerline adapters. Ill have another go when I'm home and see what I can come up with, might be an encryption issue. I'll also talk to him and see if he's made any changes to his network since this all started.
Do a factory reset of the powerline adaptors and confirm again that your neighbour does not have them. He might not be aware of what they are and is using them as a wireless access point or extender.
That's the most likely scenario, has to be that he has a WiFi extender or something of that ilk he doesn't see as a Powerline adapter. I'll speak to him when i get home.
