The coven - Graham Masterton

They say the girls were witches. But Beatrice Scarlet, the apothecary's daughter, is sure they were innocent victims...

London, 1758:

Beatrice Scarlet, the apothecary's daughter, has found a position at St Mary Magdalene's Refuge for fallen women. The charity is supported by a wealthy merchant who offers the most promising girls steady work in his factory. But when seven girls go missing, Beatrice is uneasy.

Their would-be benefactor claims they were a coven of witches, beholden only to Satan and his demonic misdeeds. But Beatrice is convinced something much darker than witchcraft is at play...