u/AgingWatcherWatching

I’m currently reading “A Man Lay Dead“ and it reminded me of another similar murder mystery, but I cannot remember the name of it.
I’m hoping you can help me. This is what I remember, and I may have it a bit muddled: the book is set on an English estate and there is a guest who is supposed to be, I think it was, a Russian prince. There is a murder and the investigator, who is not an official in the Police but who helps the detective, at one point is following one of the suspects through the property and through a gate and they discover a small area where someone could’ve hidden. I believe that the Russian turns out not to be the prince, but someone pretending to be him. He also has, I believe, a bodyguard with him.
I have tried searching and can’t find anything that matches. But I know I have read the story. Can anyone help identify it? Feel free to ask questions, I will try to remember as much as I can.