I found this book to be delightful and the ending made me laugh out loud with glee. The solution to the "impossible crime" was absurd and contrived - as these impossible crime solutions often are - but not such that I was annoyed.
I didn't guess whodunnit. I was pretty sure throughout the entire thing whodidntdunit, and I was right about that, but I focused on the wrong character.
Specifically, I thought for the longest time that Duchlan had finally gotten fed up and murdered his horrible sister. Then, towards the end, I started to suspect that possibly Eoghan's mother wasn't actually dead, but was horribly disfigured and had been living in a hovel on the loch somewhere plotting her revenge.
The victim, Mary Gregor was an odious woman. She reminded me a lot of Mrs. Boynton, from Appointment With Death, which remains one of my favorite Christie mysteries. Some people go unmourned for good reason. The second inspector sent to investigate, Barley, was a blooming idiot with a bad case of confirmation bias - he decided who did it, and then try to squash the evidence into agreeing with him.
The book did drag a bit - this I cannot deny, and I agree with Tigus that the talky-mc-talkerson grew tiresome. I was totally astonished by the THIRD murder, and by the fourth, I was dying to get to the end! Overall, this ended up being one of my favorite of the BLCC reissues.