I am making a guess here. Don't look under the spoiler tag unless you have read it!
I don't think it is either Elliott or Phyllida, because Patricia Wentworth enjoys her matchmaking and putting marriages back together way too much. I am leaning against Mark & Lydia for much the same reason.
I'm going to go with Aunt Grace, for reasons which I can't really explain. Maybe because she is angry at her brother for interfering and trying to get Elliott and Phyllida back together.