It should be the extra over cost that the CE is going to cause - which would sound like the premium time not the normally hourly rate as well.
It is for the Contractor to present their quotation justifying any additional cost they believe the CE will give them. If you don't agree their quotation then obviously you have the right to assess it yourself - to what you believe is correct in accordance with the contract. Their only formal way to overturn your decision would then be adjudication.
It is for the Contractor to present their quotation justifying any additional cost they believe the CE will give them. If you don't agree their quotation then obviously you have the right to assess it yourself - to what you believe is correct in accordance with the contract. Their only formal way to overturn your decision would then be adjudication.