Jules Jones

Yes, I remember that one, and something about the husband as Lancelot. I too worked out who did it at once... actually, I seem to remember that in the days when I read a lot of Christie I generally could tell who had done it, though that may have had something to do with the publisher's annoying habit of commissioning cover designs that always related to the solution of the mystery. (I don't usually work it out in the TV dramatisations.) But, covers aside, I don't think it's necessarily a fault if you can deduce the answer, because it's satisfying waiting for the detective to catch up.