Being born and raised in Middlesex I'm used to being mocked for drawring (and after thirty odd years of it I still don't think I put an R in it) but to me, in my ignorance, Napa and Napper are homophones. It's just the long Ahhh sound of the letter A (like Bahhth, which foriegners (i.e. not NW Londoners and surroundings) seem to add an R at the end when it's really an H.