The Contractor has not followed our information on site (we are employed as their designer under the PSC(Main contract Option C - Target) what are our obligations and how do we calculate the increased fee from this extra work? There are only a few months left on site.
As the Consultant I would be issuing an Early Warning Notification to the Contractor that he is not following your design in the construction of the works. The risk of not following the design lies with the Contractor.
You do not outline what you are doing to incur additional costs? If we assume that you have given an early warning and had a risk reduction meeting at which the Contractor gives an instruction to do something (change the design) and a compensation event notification then the change in the Prices (target cost) would be providing a quotation for a compensation event which would be on a time charge basis.