Ooooh, oh, Robin! It's one of Burnett's novels for grown-ups, and actually it's the second in a duology, though it can be read on its own since the first bit summarizes a lot of the previous book.

But if you want the other, it's The Head of the House of Coombe, and last time I checked it was available at the Gutenberg Project.

Those two are among my favorites of her work; she wrote quite a few books that no one has heard of any more. Among other things, they're a fascinating look at England just prior to, and during, the Great War. Be warned, they're very anti-German.