I've found a name for the problematic little suckers that Nissan use to hold on the 'below windscreen grille'. They are called 'scrivets'.
http://www.fastex.com.au/plastic_fas...vet_bigger.jpg
A quick google shows that other manufacturers use scrivets too, as well as owners who don't like them.
In the case of 'windscreen leaks' that are not actually windscreen leaks, these are what are responsible. Well strictly speaking it's not actually the scrivets themselves, it's the rubber seal (see pic below) that they sit in. This seal looks like a washer from above, & it is difficult to determine if the lower section is split, or even no longer there, & removing to inspect is likely to damage a good one. I imagine it is common for folks to get their windscreen replaced, then get a leak, & assume it is the windscreen seal that is the problem, when it is the scrivet 'seals'. I suspect they are an item that probably should be replaced whenever the windscreen is replaced (grille removed).
Unfortunately, of all the online suppliers I've found, none mention these 'under scrivet seals', so maybe we have to get them at Nissan prices.
My solution of using windscreen sealant in the holes (because I had it handy & didn't want to use silicon or anything which might make future removal of the grille a problem), whilst it has stopped the leak, is only a temporary (& slightly messy) solution. Problem is it seems to get softer with the heat from under the bonnet. I can imagine in hot weather it could become a problem. So I'll get some new scrivets & seals.
Does anyone here have FAST software & able to look up the part numbers & see whether the scrivets & seals have the same or separate part numbers?
Cuppa