From what I've read, Hitchcock didn't like his version either. He is reported to have said that the story was changed to give Charles Laughton (who part owned the production company) a more substantial part. Whether that was Du Maurier's main objection, I couldn't say. Unintelligable dialogue isn't confined to the BBC. When they originally screened "Mammon" in Norway there were the same complaints that the dialogue was incomprehensible, even to Norwegian speakers, so that half they through the run it got cleaned up.