Norah Lofts had a long career as a historical novelist, during which she diversified with novels of mystery/suspense/occult under the pseudonym Peter Curtis such as THE DEVIL'S OWN (1960) which was published in 1960 and was nominated for an Edgar Award - it was filmed by Hammer Films in 1966 as THE WITCHES starring Joan Fontaine (in her last theatrical film) and was republished under Lofts' name as THE LITTLE WAX DOLL in 1970, the same year that LOVERS ALL UNTRUE was published.
Although I've encountered Lofts' books many times over the years, and even, I think, perhaps read one or two back in the 1970s, only recently did I discover that LOVERS ALL UNTRUE was inspired by the notorious "Not Proven" case of Madeleine Smith - I recently re-watched David Lean's 1950 film about the case, MADELEINE, which is considered 'minor' Lean but is actually quite underrated, and this reawakened my interest in the Smith case - of course I couldn't find a copy anywhere (Lofts died in 1983, few of her books are left in print in the US and have pretty much disappeared from the shelves of used bookstores as well) but found that it's still in my public library's system (I must drive the librarians at other branches crazy with my on-line requests for books to be sent to my local branch, making them head off into the stacks for vintage books and authors that they've probably never heard of!).
4/07: Lofts follows the basic Madeleine Smith story up to a point, and then diverges from history to provide her own (and very different) fate for her Smith character, herein called "Marion Draper." The setting of the story is England rather than Scotland, and the time period, though never specified, is almost certainly late-Victorian (water-closets did not appear in England until after 1870 and are mentioned several times), and the novel provides some interesting glimpses into the social customs and mores of the time.