SPOILERS if you want to read the book, don't read my post!
I've just watched it all, was waiting until I had finished the book. There are quite a few changes but they don't detract from it too much, save the ending, maybe the husband's affair too. Ben Chaplin wasn't the man I had in my head through the book as X. I thought they would have made more of Yvonne realising that her husband had read all of her letters to X. I also thought they would show her being arrested straight after the Court case for perjury (even though it's done as more of a memory in the book) and I'm sure she did do some time in prison for it in the book. It was very good though!