It seems to me that Ngaio Marsh was using her mystery to prod and push and perhaps even slice open a particular view of England.
It's the view of those who believe themselves born to rule. The ones who see nothing odd about the term Home Counties. The ones who honestly believe that England is exceptional because they believe that they and their ancestors before them are and were exceptional.
It's also about the people who enable and sustain this point of view. The people like Nurse Kettle who believe in "degree" and who are reassured by a social hierarchy which doesn't change and in which they know and are satisfied with their place.
It's an England born of and nurtured by the stories the English tell themselves about how the world works.
The picture map that is drawn for Nurse Kettle is a good manifestation of that world view. Moonlight said it reminded her of "Wind In The Willows". I think that may be because it expresses the same England-as-we-would-like-it-to-be-if-we-were-all-good-chaps spirit. Alleyn describes the place as charming, saying it's:
'Like a lead pencil vignette in a Victorian album.'
Given the precision of Alleyn's language, I was reminded that charm is an illusion, cast to please the eye of the beholder and to hide what is really there.
We're told that Swevenings, the village name, means dream and the river's name Chyme, means yawn. Which I think is a hint that the map is a dream of England.
I think Ngaio Marsh manages to show the power of the charm and the ugliness it hides and grant both things a degree of authenticity.
Nurse Kettle, who is happily in the charm's thrall, is admirable in her way and her world view helps her to remain admirable. Lady Lacklander, who helps to cast the charm, is also admirable in her way. She plays the noblesse obligé game without reluctance and props up the dream through her unassailable certainty in her own entitlement.
Yet the reality the charm is hiding is not just one of disproportionate privilege but of treachery and exclusion and thoughtless exploitation.
Alleyn is able both to see the charm and see through it. He has dedicated himself to facts, even when he doesn't like where they lead him.
It seems to me that the biggest 'fact' Alleyn exposes is that the charm itself is toxic and damages those who cast it, those entralled by it and those excluded from it.