I'm taking it that this is a beloved children's book in England, given that it made it onto the BBC's Big Read list. I don't really see it. It was fine, but it didn't have the magic of some other great British children's books. Not a patch on Enid Blyton, for instance.
Four children convince their mother to let them stay on an island on the lake where they are vacationing for a week before they leave. Once there, they get into nautical battles with the Amazons, two girls who claim the island for their own. Eventually, they join forces, and turn against the force that has been against them - the girls' uncle, who used to play with them, but now blames the first four children for everything that has gone wrong on his houseboat that summer.
There's buried treasure as well, and a lot about sailing.
It was a fast read, but I just don't think it'll stay with me. Other than the unfortunately named Titty, most of the names are already slipping away from me.