Step 1: record it
Photograph the damage with a date, note when you first saw it and what work was happening at the time.
Step 2: tell the builder and the surveyor
Let the builder know so they can take care to avoid further damage. Tell your surveyor, or the agreed surveyor, in writing.
Step 3: inspection
The surveyor inspects and compares the damage with the schedule of condition. That is where the before-and-after record matters: if a crack was not in the schedule and has appeared during the work, it points strongly to the work.
Step 4: settlement
The surveyors decide what work is needed to put it right, and whether it should be repaired by the building owner's contractor or paid for so you can arrange the repairs yourself. See repair or payment.
If there was no award
If you consented to the notice and no award was made, a dispute about damage can still be referred to surveyors under the Act. This is where photographs taken before the work become invaluable.
Timing
Some damage, such as hairline cracks, only shows after the structure settles. Many surveyors carry out a final inspection after the work finishes; ask for one if it is not offered.
Where this comes from in the Act
Links go to the official text of the Party Wall etc. Act 1996 on legislation.gov.uk.