I read the novel many years ago, and loved it. This adaptation struck the right mood, it seemed to me, dark and subtly sinister. I didn't have problems understanding the dialogue, for the most part--yes, Joss Merlyn is a mumbler, but I remember that as being in character.
I did think some of the "night" scenes on the beach could have used better light filters--they look like it's mid morning on an overcast day! At times it did seem a little heavy-handed,and as a fan of BBC drama, I wouldn't put this in my upper tier favorites. But I did enjoy it, found the performances to be generally good (Jessica B-F made a very good Mary Yellen, who is not a simplistic heroine)and the sweeping moorland scenery was a plus as well.