I'm in SE England and always say it in two syllables: DI-mond. But I've just been listening to an audiobook read in a very RP accent, and every character gives it three syllables: DI-a-mond. The a is more of a neutral shwah sound than a flat a.
AFAIK I speak RP with a slight foreign accent at times (1st gen immigrant).
So, which is right?