It looks like you have answered your own question.
The Contractor gives an instruction under clause 17.1, which the Subcontractor is obliged to obey, in accordance with the obligation under clause 27.3.
If the Subcontractor still believes this is outside the Scope then the matter should be referred as a dispute, although probably better to sit down together firstly and for the Subcontractor to explain why they believe the work is not included in the Scope.
The Contractor gives an instruction under clause 17.1, which the Subcontractor is obliged to obey, in accordance with the obligation under clause 27.3.
If the Subcontractor still believes this is outside the Scope then the matter should be referred as a dispute, although probably better to sit down together firstly and for the Subcontractor to explain why they believe the work is not included in the Scope.