Answered: NEC ECC: Under Clause 50.1 of the NEC3 ECC Contract does the assessment interval start again once the assessment required at Completion is made or is the assessment at Completion in addition to the regular assessments made at the contractual assessment interval?

I don't think so.

The last sentence of clause 50.1 says:
"Later assessment dates occur
 at the end of each assessment interval until four weeks after the Supervisor issues the Defects Certificate and
 at Completion of the whole of the works."

This implies that that the assessment date at Completion of the whole of the works is in addition to the normal ones at the assessment interval. There is no hint in the wording  re-setting the assessment interval to be from Completion.