Probably a bit too late now to invoke a PWA and really only your neighbour can do that. you coulkd take out an injunction but better to negotiate if you are generally on good terms.
What is your concern, the same rights that are afforded under the party wall are available under common law, i.e. your neighbour has a right to use the party wall(it belongs to both of you, a PWA just formalises that right, with conditions of protection, the same protections you have within the law. If you have given your consent for them to build on the party wall they can build on any part of it, there is no your side or their side, it is a party wall. If their loft has gone beyond the party wall that is another matter.
but as I said previously if you dissent then they can dissent to you doing the same or you can get a surveyor to resolve the matter.
Personally speaking I wouldn't worry about it, treat it as wings and roundabouts, they have paid to erect the Party wall, if your own loft structure won't fit into the party wall because of their structure then consider your cost as being the alternative structure you will have to build. I have done many aloft adjacent to an existing conversion and in most situations just moved our steels over adjacent to the existing steels.
For example your neighbour puts their floor beam on the party wall, you put yours on the party wall next to theirs.
This argument has got to the point of splitting hairs and legalities rather than finding structural solutions.