The screws in question (the 2 just under the number plate plinth, right?) are a cone/tapered head and not flat like those linked above. I'd say they're an M4 size as a guess, approx an inch long, from memory.
Hmmmm, yeah I had a chinese grille and immediately threw it in the bin when it arrived
I ended up painting the original one in satin black to get rid of the 57 shades of grey.