This book is the rest of what Willis calls a Double Decker novel. Blackout was the first part. This one continues the story. There's a forward by Willis (to the audio version at least) that tells you to go back to Blackout if you haven't already read it/listened to it. But I don't see the sense in publishing them separately. As a serialized magazine novel, maybe, with All Clear having 2 or 3 parts itself, but with the intention that it would eventually be published as one novel. All this format is likely to do is irritate readers who expect each volume to have individual integrity.
That said, I read it because I am fascinated both by the idea of time travel and by London during the Blitz, so I couldn't not read this one, but I probably wouldn't recommend it to someone who didn't have one or the other peculiar interest. Willis used the Mass Observation Project documents to get the details right--and my guess is she amassed so many details that she didn't discipline herself to select those likely to make a tightly organized novel. Mass Observation was a sociological study in Britain, starting in the 30ies in which ordinary citizens kept diaries of their everyday life. I think they were paid to do so--probably compelling in the 30ies. I'm not sure there was a clearcut objective, but the result was a huge mass of material and it continued through the war. Only now are sociologists and historians and writers of fiction mining the documents--which are archived at Sussex University as I remember. (More info: http://www.massobs.org.uk/index.htm)
Willis also interviewed a number of older women who had had "war work" in WWII and who had a reunion at the Imperial War Museum in 1995. Quite fortuitously, Willis was there to doing research for the novel. She uses the reunion as an event in the novel.
So the book is fascinating because of the detail of life during the Blitz. The main action takes place between September 1940 and Spring 1941, though it begins with Dunkirk and there is some action during the V1 and V2 rocket attacks and on VE Day in May of 1945. The basic plot focuses on 3 "historians", i.e. time travelers from 2060 who go back in time to study history from the front lines. But they are unexpectedly trapped in the past when their "drops" (locations where they go to be "beamed up" as it were) unexpectedly and mysteriously close. They speculate that they've done something to change history--a no no in time travel speculation--and as a result their ability to travel in time is shut down, whether temporarily or permanently they don't know. They fear what they've done has changed the outcome of the war. They fear that the Oxford of 2060 (home to them) has been destroyed.
The plot if VERY convoluted and there's lots of sentimentality and lots of stock characters and melodramatic situations, but it's compelling enough to finish just to know what happens, though the title clearly gives you a clue. I could never recommend this as great literature. Probably those fascinated by London during the Blitz will like this better than science fiction types who love time travel. Willis never claims to construct a viable theory of time travel, but she doesn't even resolve all the questions and contradictions her "theory" raises.