I mentioned this a couple of months ago and I've got it from the library now. I've read a few pages and it seems worth pursuing and I plan to, once I've finished the other two books I've got on the go. I tend to read serially rather than in parallel, usually. (It's one of those dinky sized hardbacks too, very nice.)
Synopsis from amazon.co.uk: "It is Boxing Day circa 1935. The place is a snowed-in manor on the very edge of Dartmoor. It is a Christmas house-party. And overhead, in the attic, the dead body of Raymond Gentry, gossip columnist and blackmailer, shot through the heart. But the attic door is locked from the inside, its sole window is traversed by thick iron bars and, naturally, there is no sign of a murderer or a murder weapon. Fortunately (though, for the murderer, unfortunately), one of the guests is the formidable Evadne Mount, the bestselling author of countless classic whodunits. In fact, were she not its presiding sleuth, "The Act of Roger Murgatroyd" is exactly the type of whodunit she herself might have written."