Option E with Schedue of Rates

Currently working under an NEC3 Option E contract that contains a “Data For Schedule of Cost Components” that consists of a number of work items that have been priced as a composite rate only. The contract states that the Defined Cost is in accordance with the rates contained in this schedule.
A group of the items are banded by qty, 1No, 1-6No or 7 and above, for similar items and the UoM is per item.
The Contractor is now claiming that the UoM should be per day as the rates per item do not decrease in line with the increased banding.
I claim that the UoM can not be changed and therefore the works are to be paid per item x the rate agreed.
Can you please advise