I don't know whether the author ever explained the reason behind her shift--perhaps it was a recognition that readers were increasingly interested in series books, particularly in the developing cozy genre, which her books came more to resemble in the 1980s and 1990s. As late as 1981, she could still a manage a book as comparatively "dark" as Experiment with Death, but her books afterward usually seemed to have a lighter touch, even her non-series ones.
Ferrars had wandered rather over the track in the Forties, producing the five Toby Dyke novels, then some very serious crime novels, but Milk of Human Kindness is more of a manners mystery, a domestic comedy of murder with some echoes of HIBK from the Thirties. (See below.) Milk would set the template for many of her later domestic village murder mysteries, though in its case the author brings humor to the fore.
Norman Rice, a kindly if somewhat austere retired colonial civil servant engaged in writing a never-ending book (like Andrew Basnett in the Andrew Basnett series)
Norman's daughter Beryl, now learning gardening professionally, and Maurice, a day student somewhere
Basil, the Rices' amiable and smoothly competent cleaner ("He looked about twenty-five, was not very tall, was built with extreme neatness and grace, and had curly golden hair and blue eyes. It was a little difficult to believe in him.")
Ernst and Millie Weinkraut, the Rices' neighbors
Later on Susan and her third husband, Piers Beltane, show up too. All in all, it's a cast of characters (and murder suspects) that would not shame a Thirties country house mystery, though we are lacking a butler, of course. Ferrars really manages to keep the plot twisting and turning. There are three murders and a denouement which produces three different solutions: one from Marabelle, one from the stolid police inspector and then the one which is actually right.