Lionel West He is a passionate stamp collector who knows all about stamps. He told Miss Marple about rare and expensive stamps including an 1851 blue 2-cent stamp which fetched $25,000 at an auction. This helped Miss Marple solve a mystery in Strange Jest.
Lionel is described in the text simply as "my great nephew". It is not entirely clear that his family name is "West". If it is assumed that Raymond West is Miss Marple's only nephew, then it is a fair assumption that Lionel is Raymond's son. In 4.50 from Paddington, Raymond's second son is named David hence Lionel could be the eldest son.
In the anime show Strange Will, NHK's adaptation of the Miss Marple short story Strange Jest, Lionel is also mentioned in the same context. Here he is also a great nephew but in this case he cannot be the son of Raymond West, otherwise he would be the brother of Mabel West. Mabel is listening along as Miss Marple mentions Lionel but does not appear to know him.