In the south of England, we say 'grarse' for 'grass'. In the north, they say 'grass' rhyming with 'ass'. Same goes for glass, class, mast, etc. Americans, in general, also use the northern method, which was the 'original' English method. The southern 'long a' sound is only a few hundred years old.
I want to know where the dividing line is in the UK. Where does glarse become glasss, and grarse become grasss? I think Brummies use the southern style, but once you hit Warrington, they don't. Matt, what do they say in Stoke? Specifically, if you live in the Midlands, or know people up that way, please reply!