The question about whether you pronounce 'grass' with a short A or a long A is bollocks. That's a regional thing, not a class thing. A working class person from London or the south east would say 'grass' with a long A, for example, while an upper-middle class person from the north of England might easily pronounce it with a short A.
Anyway. I got 16% and presumably would have scored even lower without the flawed logic of the pronunciation question