NEC ECC: At what point does the PM assess cost of correcting Defects under Clause 45.2

If the Employer chooses not to grant the Contractor access after Completion has been certified to correct a Defect as per clause 45.2, when should the Project Manager assess the cost to the Contractor of correcting the Defect?

Should the PM assess the cost immediately and  deduct the amount from the next payment assessment or should he wait until the Defects Certificate has been issued to deduct the money?

Option X16 has not been used so there is no Retention.