Not all works are notifiable under the Act. In the first instance we suggest that you try to speak to your neighbour or otherwise ascertain detail of precisely what works are being carried out and whether they are notifiable under the Party Wall etc Act 1996.
If the works are notifiable under the Act, and notice has not been served and the procedures set out within the Party Wall etc Act 1996 are not being followed, then the works are unlawful and the building owner is in breach of their statutory duty.
You could apply for an injunction from the County Court to stop the work until such times as the provisions of the Act are complied with. If you do, you will likely be asked by the Court to provide an undertaking as to costs (which means that if the works are found to be non notifiable, or if the Court decides that your request for an injunction was not appropriate, you may be required to pay substantial costs including those of the building owner).
Except in the most extreme circumstances (such as significant and costly damage being caused by unauthorised works) we would suggest that you obtain legal advice prior to seeking an injunction, and you should always attempt to speak to your neighbour to understand their position and attitude towards your interests.