NEC ECC: Should CE be based upon actual or forecast if now known?

NEC3 Option C Contract unamended for CEs:
If time has elapsed since the instruction and it is subsequently known that the content of a revised quotation is incorrect, e.g. it includes resources that were forecast to be used, but in fact have not been procured should the quote be assessed based on the actual resource level or the forecast resource level?

You will find the answer to this in clause 63.1. It should be based upon the actual defined cost of work done, and the forecast Defined Cost of the work not yet done. It goes onto say the switch point between the two is when the instruction was given, or for all other events when the compensation event was notified (and NEC4 gives this a name of the “dividing date”. This means if instructed (and no work should have done before the instruction) then it will only ever be based on a forecast as to what would have been reasonable to have allowed.

We do not just revert to actual cost if the quote has not yet been agreed and the work has been done. For one, of the Contractor took longer than they should/could have, then the PM can assess it as to the output/cost they believe they SHOULD have achieved, not what they actually spent.