Hello, welcome to the website. My name isXXXXX can assist you with this.
If there is no mention of what they're proposing to do in the Party Wall Act agreement, then they cannot do it. Was this dealt with by agreement between the parties or by a formal award by a surveyor?
By agreement between the parties
Okay. Then can you reach agreement about this do you think?
It seems that there are two grounds on which you could challenge what they're proposing to do. (1) that you have no agreed it in the party wall agreement and thus it cannot be done and (2) that it would amount to a nuisance and interfere with your reasonable enjoyment of your property.