The Clock Strikes Twelve  - Patricia Wentworth

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. 

(show spoiler)

 

 

We'll see!

 

Aaaah! I'm already second guessing myself!