I also found that there are three short stories by her up on Belgravehouse.com, so I have downloaded them. I have not read them yet, as I only just found them, and, besides, I am in the middle of rereading Jonathan Strange and Mr. Norrell, but I have them.
So the book: Superfluous Women Daisy Dalrymple #22 (wow!) is coming out in June, 2015, and is a proper country house, locked room mystery.
The official description from Goodreads:
In England in the late 1920s, The Honourable Daisy Dalrymple Fletcher, on a convalescent trip to the countryside, goes to visit three old school friends in the area. The three, all unmarried, have recently bought a house together. They are a part of the generation of "superfluous women"—brought up expecting marriage and a family, but left without any prospects after more than 700,000 British men were killed in the Great War.
Daisy and her husband Alec—Detective Inspector Alec Fletcher, of Scotland Yard —go for a Sunday lunch with Daisy’s friends, where one of the women mentions a wine cellar below their house, which remains curiously locked, no key to be found. Alec offers to pick the lock, but when he opens the door, what greets them is not a cache of wine, but the stench of a long-dead body.
And with that, what was a pleasant Sunday lunch has taken an unexpected turn. Now Daisy's three friends are the most obvious suspects in a murder and her husband Alec is a witness, so he can't officially take over the investigation. So before the local detective, Superintendent Crane, can officially bring charges against her friends, Daisy is determined to use all her resources (Alec) and skills to solve the mystery behind this perplexing locked-room crime.
It sounds like this will be very much Daisy's story as Alec cannot be the lead investigator and so cannot take the story galloping off in some other direction the way he did in Anthem for Doomed Youth Mind, I liked Anthem for a Doomed Youth, and I like Alec, but I have read twenty-one books because Daisy is the heroine, and I don't want to see the series becoming the Alec Fletcher mysteries. He can have his own series, if he wants.