Well, I went and did it....got an Aero Grille and replaced the MINI One stock grille.
Upper was no problem at all - bolt off, bolt on.
Lower took a bit more work
Removed bumper, took off chrome surround and black strip. Cut down the lip on the Aero Grille all the way round. Cut out the grille support from the bumper itself allowing the aero grille to sit flat (otherwise the mesh fouls the raised support). Got it test fitting just right and then epoxy bonded it to the bumper - perfect fit. Unlike others I have seen this does not leave the top part jutting out at all. It is as well fitted as on the aero kit itself.
Very pleased with myself....