Polly Churchill is worried. She's stuck in London in 1940, and the handy time-travel doohickeys that are supposed to take her back to the mid-21st century aren't working. She's afraid she, or her friends, may have changed the space-time continuum and caused the Allies to lose World War Two.
(I am indebted to TV Tropes for giving me a piece of terminology to understand this phenomenon: Godwin's Law of Time Travel - "As the amount of time-traveling you do increases, the probability of Hitler winning World War II approaches one." )
Polly is, of course, being silly. As she would have known if she'd read To Say Nothing of the Dog and Doomsday Book: historians can't change the past like that, even if they try. The past remains the past, even when you're in it. Changes can be propagated forward, only if they're so insignificant that nobody from your own time would have noticed them - you can, for instance, place some newspaper ads in the 1940s, which may then be spotted by your colleagues 120 years later. But you can't change the course of known history.
Polly may be excused for not knowing this - she's only a postgrad student. For James Dunworthy, however - the head of time travel research in Oxford throughout the 2060s - there's no excuse. Why, after two full-length novels and several short stories, he should still be propounding this nonsense about "the continuum breaking down" is beyond me.
This train of thought somewhat mars my enjoyment of Connie Willis's marathon opus, Blackout/All Clear. Which is a shame, because there's a really good book in here somewhere. (Only one, mind you. The 'two volumes' thing is quite unnecessary.)
Connie Willis does her usual stellar job of establishing sympathetic central characters, a strong supporting cast, and the fretful, fatigued atmosphere of wartime Britain. She's done a lot of research, and it shows. She has, however, left some irritating holes in her blanket of authenticity. For instance, her British characters consistently give dates in the modern American format (e.g. "May fourth", rather than "the fourth of May" or even "May the fourth", either of which would be more natural).
Willis wanted to write a book about the everyday heroism of ordinary people that won the war:
about Dunkirk and ration books and D-Day and V-1 rockets, about tube shelters and Bletchley Park and gas masks and stirrup pumps and Christmas pantomimes and cows and crossword puzzles and the deception campaign. And mostly the book's about all the people who "did their bit" to save the world from Hitler...
And at that, she's done well. Her account of wartime Britain is detailed, sympathetic and absorbing. If viewed as a collection of narratives about people who experienced evacuation, blackouts, shelters, rationing, Dunkirk, the Blitz, the V1s, and the daily expectation of loss and grief - it's a masterpiece. Where it falls down is in the time-travel narrative that ties these little human-interest stories into a coherent tale. While that story has its high points (mostly centred around St Paul's Cathedral), it's much, much too drawn-out to be sustained by its basic premises.
The result is that, when I'd finished reading, not only were my withers (quite sincerely) wrung by the prolonged hardship endured by my forebears, but I felt I'd accomplished no mean feat of endurance of my own. Which feels like an unworthy thought, but that's the price of honesty.
Overall rating: B-. Good, but should have been better.
 Incidentally, if anyone with a TV Tropes login is reading this: the above-linked page, at the time of writing, incorrectly states that events in To Say Nothing of the Dog cause the Nazis to win the war. This is completely untrue - that fear is raised at one point, but the whole point of the book is to demonstrate just how resilient history is - the ridiculous lengths it can go to, if necessary, to make sure it turns out right.