Let's take this question in generality first : your contract is deliver to deliver the 'works' (in italics). So, to take a real life example, if the 'works' were for 'The construction of X' and then the Employer wanted to add in design, you would have a very good argument for saying something like "No, the scope of my contract is for 'construction of X' and does not include design, so I don't have to do it."
However, in the legal interpretation of a contract, detail over-rides generality, so if the original Works Information (the detail !) already had some design in it, you would not be able to sustain an argument that you didn't have to do the pre-existing design. If the PM instructed new work of a similar nature was introduced for which the Contractor had to do similar amount of design, then the Contractor would probably still have to do it. But if new work was introduced which had greater levels of design or was of a different nature, then it would become more subjective.
In your subcontract, from what you say, it seems that the Contractor is instructing work for which there were specific exclusions so the understanding of the Parties was that these were NOT within the definition of the 'works'. But the big Question for me, is what legal status do these exclusion have i.e. were they written down ? In which document - form of agreement, subcontract data, Works Information (Contractor's or Subcontractor's) etc. Without knowing this, it is hard to give a definitive answer.