DNO's responsibility ends at outgoing side of cut-out. If wrong polarity in installation, they would check at cut-out. The reason they turned up quickly is that they take reversed polarity very seriously. We were expected to be on site in 1 hour. If incoming polrity is wrong it needs to be corrected by dno. This would be taken seriously. If it was the installation connected wrong way round then I would have not corrected it, that would be down to supplier or electrician. Dno operatives are not allowed to do any work on the installation for obvious reasons.