Thanks for your reply, very helpful but I am being stupid I think so maybe you could clarify something for me;

You are not telling me to manually change the ISO on my camera to 12, correct?

You are saying that I should compose as if I was taking a normal shot, pop in the filter and then decrease the shutter speed by 4 stops and also bracket by one stop as well, correct?

Or could I manually change the ISO to 12 and take the shot as the camera sees fit whilst also bracketing for safetly?

Thanks