Help masking sky...

I'm reasonably good with the refine mask tool in cs6 but this image is presenting some difficulty. I'd like to mask the sky and replace it with a darker sky.

I've tried a number of tools including the refine edges, select color range and haven't been able to refine the part of the mask around the tree branches on the right, always leaving some white fringing.

Can anyone suggest a technique which will work better for what I'm trying to do? Seems like this ought to be pretty straight forward.