I have heard people mention that sometimes Raymond's wife changes from Joyce to Joan or it might be the other way round. I thought Grizelda's son wasn't born until after Body In The Library.

No, in Chapte 15 of The Body in the Library Miss Marple goes to the vicarage to ask if there is any fund raising going on so that she has an excuse to call at Basil Blake's house and talk to Dinah Lee. On entering the vicarage drawing room she finds Griselda  playing with baby David who is crawling on the hearth rug. By the  time we reach 4.50 from Paddington David has become Leonard and is home from boarding school for Christmas.

leonard is the vicar's nephew (murder at the vicarage). david is the vicar's son (4:50 from paddington).  david also appears (as an infant) in "body in the library"

I haven't read these two books in ages, but I can say that Dame Agatha DID indeed change the names of a few characters from book to book.  Raymond's girlfriend/fiancee was originally "Joyce" in an early book but later his wife was referred to as "Joan".  Speculation is twofold.  Either Raymond was with two women, dating Joyce, then later marrying Joan, or an error was made by Agatha Christie and no one caught it.  Thus, Raymond's Mrs. became "Joan" permanently.  There are other examples but I chalk it up to not keeping a character index and/or poor editing.  "Let's get Marple to the presses ASAP.  We need them on the shelves before Christmas."  That sort of thing.

Having re-read Murder at the Vicarage I discovered that the Vicar's nephew is called Dennis so back to square one.

susandiane

leonard is the vicar's nephew (murder at the vicarage). david is the vicar's son (4:50 from paddington).  david also appears (as an infant) in "body in the library"

I'm pretty sure Dennis was his nephew in Murder at the Vicarage which I recently finnished reading. Then again I could be wrong.