Whilst I do not disagree with Jon’s response I have a slightly different angle on this question/answer.
My first point if you hadn’t mentioned about withholding payments then I would say in this instance how does it matter how they are being communicated as you now know about them and are obliged to correct them? A Contractor is obliged to correct a defect whether they have been notified or not. I do think that clause 13.7 can be administered too specifically without a degree of common sense. If there were 300 small defects identified – do these really have to be notified separately? Much more practical would be to send one communication stating please be aware of the attached list of 300 defects that need correcting.
The second point would be that completion of an activity schedule does not have to be “defect free” to be paid. If as a Contractor an activity schedule item was to “install pipework from A to B” – once that pipework is installed I would expect to be paid. The fact the pressure test has not yet been done on it is irrelevant, along with the fact that if one of the valve handles has been identified as damaged and needs to be replaced (but can be replaced without removing the whole valve). Nowhere on the activity schedule does it say an item has to be defect free to be paid – and the Employer has section 4 of the contract to protect themselves that any defects have to be corrected by the Contractor.