(Parenthetically I see why they changed the name to The Year's Best Fantasy and Horror in short order, because half the stories at a guess are already that.)
So I read it, and a brilliantly unpleasant read it is too. But I'm someone with an anxiety disorder, apt to find horror lingering in the oddest of places; and I wonder why it works for people who aren't? (Just as, for whatever reason, I never had the first clue why anyone would find The Turn of the Screw scary. MR James' odd things seen out the corner of the eye-- hell, the whole tatty late Victorian setting of James' stories-- are instinct with menace to the point I cannot read them. James? As frightening as Lovecraft, which is to say, risibly not at all.)
The horror in Harrison's story, for me, is caused in great part by the English setting: because all of England gives me the fantods now. I have no idea why. Streetview, photographs, even the English landscape paintings featured at First Known When Lost, carry an unplaceable but sharp sense of unease. 'This place is not *right.*' 'Something terrible is about to happen.' Exactly as Pam in the story finds the physical world about her-- not right, not friendly, falling into chaos. No surprise when that segues into odd glowing things in the passageway.
My antidote BTW is to think of Tokyo: an unreal city, but not menacing in the least.
(Googling around I find Harrison himself saying that the novel owes more to Charles Williams than Arthur Machen. I can see that: but Williams always tips his hand; you know where you are with him; and with Harrison you don't.)