Woo yay, the grille fits and it looks great. Can't really see the fixings from a distance, which is nice. I ended up using the edge trim around the edge of the GRP before fitting the grille from behind. Makes for a much tidier appearance and is good in that the grill itself can be changed really easily - 4 tywraps and it's done!
Not sure of the impact resistance of the material (it's aluminium) but time will tell if it'll need replacing soon.