I'm doing this from memory, so some of this may actually be book 3 - apologies if so!
Edward ditches Villy for Diana at last, Villy takes it very badly. Before Edward ditched her he buys her a new house (that she doesn't like) and she moves into it with Miss Milliment, Lydia and Roland.
Teddy comes back from his National Service married to an American ten years older than he is, who runs off back to the States when she realises he isn't as rich as she thought he was.
Polly works for an interior design company who treat her rather badly, but send her off to give some advice to a young man who looks like a frog and has bought rather a grotty flat. That is Gerald, who later turns out to have inherited a title and a really hideous stately pile. Cousin Christopher, who fell for Polly very heavily, goes to help out at his sister Nora's home for disabled ex-servicemen, and after many conversations with the local parish priest sets off to try his vocation as a monk.
Clary falls heavily for her employer, Noel, who is clearly a prize shit. Eventually the inevitable happens and she gets pregnant; Noel and his wife Fenella don't want anything further to do with her and she has an abortion, which Archie supports her through. After that she falls into depression and Archie helps her through it by renting a cottage, installing her in it and encouraging her to write. She writes her first novel and in the course of it falls in love with Archie, who has been in love with her for the whole book.
Louise separates from Michael Hadleigh and leaves her son with him to be brought up.
Sid has an affair with a young woman who came to her for violin lessons. Rachel finds out and is devastated, but after the Brig dies she and Sid are reconciled.
Simon is still doing National Service by the end of the book, but is thinking of doing medicine when he gets out.
Hugh is still grieving dreadfully for Sybil when the novel opens, but by the end has met a young war widow (with twin sons) who has come to work at Cazalets as a secretary and married her.
Rupert and Juliet each confess to the other that they have loved someone else during the war, and start to rebuild their relationship.