NEC 3 Option B - Defects

A Defect is notified that part of the works is not in accordance with the Works Information. The Contractor subsequently disputes this on the basis that he considers it to be a design defect (Employer design).
For the case in question, searching for the defect is unlikely to be conclusive either way.

Is the Contractor obliged to correct this defect as notified?

I’m not sure what sort of defect cannot be proven one way or another, it’s most unusual. The Supervisor should instruct the Contractor to search if they suspect the works are defective but can’t prove it, a Defect shouldn’t have been notified until liability was established. If you dispute the Defect and therefore don’t correct it you do run the risk of the PM assessing the cost of others correcting the defect and the money being deducted from you. If this happens your only formal recourse is adjudication which will be expensive and time consuming. If you haven’t done it already you could write to the PM stating that you don’t agree with the Defect and explaining in detail why. Whilst there’s no contractual basis to do this (i.e. the contract doesn’t require a Defect to be “agreed”) it might help to change their minds.

As an alternative you might want to propose to the PM to accept the Defect under clause 44.1 (even though you don’t agree that it is a Defect!) and basically use this as a way to negotiate either no cost to the Contractor or sharing the cost between you.