The Brits I know say "mah-nah." The Yanks say "man-na." It's all that twangy flat "a" that we Americans use.

I've always thought it really strange that one of the few words that Americans pronounce with the longer, softer "a" -- taco -- is a word that the Brits shorten the "a" on. So most Yanks say "tah-co" and Brits say "tak-ko". Go figure.