The Project Manager gives an instruction adding additional work to the project and at the same time requests incorporation into the programme, which states constraints on when the additional work must be completed.
There are no key dates or sectional completion dates.
A Project Manager can try - but there would be no natural contract remedy if you then fail to meet that date. Key Dates have to be tied into the contract and can not be imposed mid-project.
I understand this will be a difficulty for the Employer here - but it should be resolved by agreement and incentivisation rather than trying to shoehorn in a penalty if it is not achieved as I see no where in the contract that allows that to happen.