Hunting down some of the paperbacks that I read as a kid, trawling through my parent's archives.
The story takes place somewhere in the British Isles, and the main character is an artsy-fartsy type, likely in the 70s. He has a sort of ongoing antagonism with a more straight laced member of his community about the same age.
Parts of the novel are told from his perspective. The other sections are told from the POV of an alien being living underneath the henge nearby. It's a massive tentacled thing that was ostracized from its home, and has come to earth to perform an experiment.
Both our main character and his antagonist are constantly reincarnated creations of the alien under the henge, with the goal to see who solves the mystery of their existence first. What has happened every other iteration is that the straight laced fellow eventually murders his artsy-fartsy counterpart, ending the game in a draw.
I may be misremembering some details, and I would not be surprised to discover the book was very bad - in retrospect I can see a sort of thinly veiled pro-hippy, anti-Thatcher agenda laced with some ancient aliens nonsense, but I've never been able to hunt down a copy to check my recollection.