-
You're within your rights to grass your neighbours up to the council planners, as they've deviated from the plans against which their PD Certificate was granted. But this would only serve to destroy your relationship with them, and unlikely to achieve any change as what they've built does seem to still fall within PD.
Also, unless you've given explicit consent for modification of your property as part of your party wall agreement, then they've fallen foul of that as well. Depending on how your chat with the neighbour goes, I'd suggest getting a surveyor out at their expense to assess the situation.
Happily, it is in the public domain on the planning website, so see here attached.
I am planning on opening a dialogue about it. It's filling me with anxiety though, so I want to be loaded with all the options.