Well, I just listened to this story yesterday, as I work my way through the older episodes, and I wanted to express my pleasure at such a classic style horror piece. I must say I'm not familiar with the names of Ramsey Campbell or Robert Aickman, though I may know their work or works influenced by them, but this story feels like what I think of as a sort of "gothic" horror, or something akin to how Poe wrote.
No, it is not subtle. And I like subtlety, but perhaps not all the time. Much like the 1980 movie The Changeling, starring George C. Scott, which was also very unambiguous in its creepiness, it quickly disarmed me of the notion that I should expect something else, and that allowed me to enjoy it for what it was. I see this story as a feast with all the trimmings -- those trimmings being the vivid descriptions, and the many, many touches that made the danger so imminent to Sara, and the suspense resulting from the fact that she does nothing to stop it. Seemingly a victim of her social inhibitions and manners, more than a paralysing fear, perhaps. I'm not sure if that's enough to explain her lack of action even after Standish's wife explicitly warns her to get out, though, and perhaps the story would have done better without that bit, so that she may yet doubt whether the danger is real (more Hitchcock-like that way, I think).
No one seems to have mentioned the dialogue, which I thought was well-written and characterised them nicely.
In the end, this story delivered exactly what I felt it promised at the beginning. I rated this story 4 out of 5 stars, which on my scale means that I enjoyed it such that I would gladly listen to it or read it again.
Robert, I would like to take you up on your offer of the text of this story, and a PM will follow in case you miss this post.