Thanks. Yes, having been doing a bit of Googling, it seems that those are the two main methods of detection of grid power loss. For example, look at this one .The Loss of mains mainly seems to be suggesting vector shift & rate of change of frequency as the methods used
As you will see, in their 'Conclusion 2' at the bottom, they admit that neither of these methods will detect mains loss in the sort of scenarios I have been postulating. They say that, because of this, DNOs require a "minimum import or minimum export", but I don't really understand that. I can't see how an individual inverter could possibly know whether it was exporting to the grid or to other local loads. Nor do I really understand how it will know about any 'importing' but, even if it did, I'm not sure how it would know whether the import was coming from the grid or other local inverters. As always, I must be missing something!
It looks like I need to continue my reading!
Kind Regards, John