Assuming you have instructed this additional works which is a change to Works Information then if you do not agree with their resultant quotation then you can assess it yourself as the Contractor. If you have instructed them under 27.3 to do this work, and then request the quote under clause 61.1, then in the meantime they are obliged to proceed with the work (even though the quote is not yet agreed).
Under clause 63.1 you would assess it based on a forecast as to what would have been a reasonable cost in terms of rates, outputs etc. This would then be "implemented" at your assessment, and they would have to challenge that through adjudication if they did not agree with your assessment (so your assessment has to be solid and one you are confident the adjudicator would agree with if it ever went that far).
Under clause 63.1 you would assess it based on a forecast as to what would have been a reasonable cost in terms of rates, outputs etc. This would then be "implemented" at your assessment, and they would have to challenge that through adjudication if they did not agree with your assessment (so your assessment has to be solid and one you are confident the adjudicator would agree with if it ever went that far).