As this is a civil contract, you do not have to 'prove'. It is on 'the balance of probability'.
So the Supervisor asserts from a broad brush inspection that the service run has become corroded due to the build not being watertight and notifies a Defect. In the absence of any counter-evidence, there is a Defect.
Your M&E specialist does a more detailed inspection and concludes otherwise. I.e. there is stronger counter-evidence, so the balance of probability swings in your favour.
The Supervisor needs to do a similarly more detailed inspection and hopefully come to a similar conclusion as your specialist. If this is not the case, then I suggest a more specialist M&E inspector is called in to look at a range of a matters. Note that if they find in the Supervisor's favour, then you will have to pay under clause 40.6 (NEC3) or 41.6 (NEC4).
So the Supervisor asserts from a broad brush inspection that the service run has become corroded due to the build not being watertight and notifies a Defect. In the absence of any counter-evidence, there is a Defect.
Your M&E specialist does a more detailed inspection and concludes otherwise. I.e. there is stronger counter-evidence, so the balance of probability swings in your favour.
The Supervisor needs to do a similarly more detailed inspection and hopefully come to a similar conclusion as your specialist. If this is not the case, then I suggest a more specialist M&E inspector is called in to look at a range of a matters. Note that if they find in the Supervisor's favour, then you will have to pay under clause 40.6 (NEC3) or 41.6 (NEC4).