This is where the 'are we still in Kansas?' feeling began for me. Elizabethan Dukes *might* have had follies with classical statues (though topiary seems more likely: and I had more than a frisson of Green Knowe throughout the book) but would they have had mermaids and sea serpents as well? Sounds much more like a left-over amusement park a la Spirited Away. The author is American; is that why he has boy and grandmother feeding 'crackers' to the ducks and growing tomatoes in their garden? And if boy has been sent away from the Blitz, why are his mother and baby sister still in the capital? Also- only one character is given a name, which I read as French because I would. Everyone else is an initial and a dash. Only one place has a name- Wool Island- which we're told isn't its real name, just what the villagers call it. This anonymoty removes the action from too definite a here and now (or there and then, if we're talking WW2 Britain.) It's in the timeless place which is childhood.
Understand, I noted these things in and around the plot events and the boy's attempts to solve the riddle of the stone garden, which touch Borgesian and Eco-ish echoes and are delightful in themselves. The ominous intrusions of an unlikable Major into village life didn't go where I was afraid it would because this is another kind of book, unclassifiable in my experience.
The afterword of course solves the book's riddle so that incongruities now make sense. No, it's not Haw Par Villa, or even close, but- well, parallels again. The learned will already have figured this out for themselves, but I am happily not learned.