I'm not sure Chris, but aren't the barrow references 'non-specific'? Is Maiden Castle referred to by name? (I can't remember.)
Like K I wrote a (shite) dissertation on Hardy's stuff as part of my degree finals, but unlike K, I'd still rather hack me head off with a big knife than read em again (not because they're not great, but because I got so fed up with em!).
I don't remember any references in his poems, but it was a long time ago that I read em & I didn't like many/any.
I read 'Sarum' that someone has mentioned & quite liked it (though I seem to remember I found it more & more boring as it moved thru history).
I recently(ish) started reading a horror novel that involved Stonehenge but I suddenly decided I didn't like reading horror stories anymore. Weird - it was just like that. I enjoyed the Cornwell 'Stonehenge' that Spaceship Mark mentioned.
I also started reading a mystery or horror book set in the Peaks that involved a stone circle a few years ago but got distracted by a new book by someone I really liked. I'll have to see if I can find it again - think I've still got it.
Finally, there're references to (semi) fictional standing stones & barrows etc (and a 'bog body') in some of Phil Rickman's stuff (which altho it could be vaguely called horror, I still like!) Mind you there's probly hundreds of books from the supernatural thriller/horror side that do that.
love
Moth