Assuming the correct process has been followed by the PM requesting a quotation and a revised quotation; and the contractor has responded accordingly, what redress has the PM got should he/she disagree with the contractors assessment. Bearing in mind the practicalities of keeping the job moving along
Project Manager should make their own assessment if they do not believe the quotation to be correct. Assuming these works have already been instructed under 61.1, the works should be proceeding anyway. The process you describe here is a parallel process to see how much the Contractor should be paid, and what effect (if any) there should be on Completion Date.
If the Contractor does not agree with the assessment/implementation by the PM then their recourse is only through adjudication.