Under an ECC NEC3 contract does the Contractor have to follow an instruction from the Employer to do extra work after Completion has occurred?
This question has been asked previously, see link below;
Notwithstanding the time issues, as mentioned by Glenn, it is likely that there are limited resources retained on the project, probably not sufficient to undertake any significant additional works. This would probably inflate the cost, which you could discuss with the PM, for example employing resources full time for the duration to manage the additional works, without the cost being shared over other activities.
As Glenn has also sensibly suggested, review the insurance provisions too, as well as confirming any access requirements to undertake the additional works.