The Juniper Tree, by Barbara Comyns

In their idyllic garden, Gertrude and Bernard forge a perfect triangle of friendship with Bella, the scarred mother of an illegitimate child. Then Gertrude conceives the child which has long eluded her, and the spell breaks into foreboding, menace and madness. (Goodreads summary.)

Comments from Goodreads reviewers:

Zuska:
"This modern retelling of the classic (Grimm's) fairy tale is full of creeping horror, and shows why fairy tales have such lasting power."

Ali:
"Comyns is not unlike Angela Carter, their fiction is often unexpected, there is darkness, magic, their worlds seem very slightly out of kilter while being entirely recognisable. Having said that, Comyns is absolutely and entirely herself..."

Rachel Jones:
"Barbara Comyns is very English. And very twisted. I love her writing. Spare but compelling."