It takes a good actor to narrate/read diary entries convincingly in films, because interpretations of characters' personalities, as with people in the real world, so typically comes from viewing their interactions. I thought that Dench mastered this extremely well. (I have also read the author of the book say that she sounded just as she imagined Barbara to sound, which is the prime possible endorsement.) I think it's a shame that the trailer really gives away the whole storyline, though I don't think this affected my enjoyment.
Small query: According to Sheba, when is Steven's birthday going to be? If you are yet to see it, could you please keep an ear open for this (towards the end)? I am pretty sure I know when she says, but I would just like confirmation. Thanks.