King of the fictional diaries has to be Diary of a Nobody by George and Weedon Grossmith, written back in the 1890s. The premise sounds deeply unpromising: the life of a clerk in Victorian London. But boy, oh boy, it's funny. Utterly English, fascinating in its revelations of Victorian life, but most of all, one of those books that you wish you're still to discover. I love it, and it's hero, Charles Pooter, a man whose personality can be summed up in the entry, "I left the room with silent dignity, but caught my foot in the mat".
The Diary of A Provincial Lady by EM Delafield (see previous posts for more on this book)
Mrs Miniver by Jan Struther - more mid-twentieth century afternoon teas
The Hens Dancing series by Raffaella Barker - modern, eccentric, but with the same vibe
The Dulcie Domum books by Sue Limb -a late 20th century Mrs Miniver
And the ones I can't bear to read, because I couldn't stand it if they weren't as good as the ones I've listed above:
The Bridget Jones diaries, by Helen Fielding.
Let me know if you have any more recommendations.