If a compensation event has been agreed and implemented under an option B contract i.e. new rates and quantities included in the BoQ. Are these items then subject to Cl 60.4 if it comes to light that one of the quantities included by the contractor in his CE quotation was significantly under estimated.
In short no. Once a CE is implemented that is full and final. An under estimated rate does not constitute a new compensation event (and equally nor can the Employer have money back if rate was way over your actual cost).
The only way this would be a compensation event is if the actual quantity of this element changed significantly after it is implemented, but then only the element that is more than 0.5% of the total of the Prices would be subject to consideration.