Contractor issued instruction for x works. Subcontractor issued quotation back. Contractor accepted quotation. Subcontractor has returned to say they have forgotten to include fee.
Where does the Subcontractor stand? The CE hasn’t been claimed or paid yet.
If the Contractor notified his acceptance of the quotation then the CE is implemented (see cl. 65.1).
Unfortunately, it is up to the Contractor’s discretion to allow the correction - it wouldn’t harm asking and you would owe them one if they make a mistake in the future.
Mention clause 10.1 (spirit of mutual trust and co-operation) - good luck!