A rule of thumb is that the best curry comes from the foulest establishments, with dank toilets, peeling wallpaper and lighting so low that you can't see what you're treading in.
Seriously, we've just picked the venue for a works meal out based on this - there are two Mexican restaurants in Swindon, of which La Casa scored 0/5 and Chiquito's (yeah, a chain, I know) scored 5/5.