If the building works known as ‘notifiable works’ under the Party Wall Act are undertaken without following the Act. you are breaking the law. Any works commenced are therefore being conducted unlawfully.
If works commence without the agreement by the Adjoining Owners you can obtain a Party Wall Injunction which will stop your neighbour from continuing with works that are currently being undertaking without an agreement or Award in place.
You should always seek Legal Advice before progressing injunctions.