As Neil has said - if this is a change to Works Information then this will be a compensation event and you should follow the notification/quotation/assessment/implementation processes within section 6 of the contract.
Just a separate word of warning though. Option A assumes that you the subcontractor produce an activity schedule. Errors in the activity schedule are the subcontractors liability, as if the Works Information says install "X" and you have not priced for it then you have to do it but will not get paid. The fact that your Contractor produced the activity schedule does not transfer that liability to them. If there were things missing you should have pointed that out/added them. Hopefully that is not the case here - but one to be mindful of for future contracts.
Just a separate word of warning though. Option A assumes that you the subcontractor produce an activity schedule. Errors in the activity schedule are the subcontractors liability, as if the Works Information says install "X" and you have not priced for it then you have to do it but will not get paid. The fact that your Contractor produced the activity schedule does not transfer that liability to them. If there were things missing you should have pointed that out/added them. Hopefully that is not the case here - but one to be mindful of for future contracts.