Christie Book Characters Named Janet

I'm trying to ascertain what Christie books contain characters with the name Janet. A friend of mine in Devon had a grandmother who worked for Christie at Greenway. She was "minding" a girl named Janet, from whom the name for a character in one of Christie's books was created.

I'd love to pinpoint for my friend which book has a character named Janet. Previously, I erroneously inquired thinking the name was Jane, and my friend corrected me.

Thanks so much.

Paul S