CE impact on planned completion ahead of contract completion (NEC4 - Option C)

We currently have a planned completion of mid March 22 against a contract completion of late April 22.
We intend to agree a CE for a further tranche of works (beginning Dec 21), taking contract completion to late August 22.
The contractor believes they are entitled to the full agreed prelims for the time between planned and contract completion, plus the new prelims allowances for the new tranche of works. Given the Pm has already ‘paid’ for the prelims during this 6 week period, we do not believe there is justification for further uplift of costs?
Thoughts?