A sad state if affairs David. If I understand the layout correctly, the architect should never have drawn it like this, and the builder is an idiot for following the plans to the detriment of simple common sense.
The dormer should have simply been constructed a few inches shorter to avoid any of this. Easy to say now.
I'm afraid the position of the boundary line is immaterial. Your neighbour's roof has been there (I guess) for many years. Long enough to have established the "status quo".
I really am sorry, but there is no way that either you or your "agents" can interfere with another's property.
Davebro is quite right about the airspace. But that is more concerned with projections made as an extra, after the house was built.
As it stands, your neighbour's "oversail" is established because it's always been there.
I'm sorry to be doom-laden about this, but I really can't see how you could legally do anything.
It's an awful situation to be in. I just hope your neighbour doesn't decide to take it further.
I wish I could be of more help.