The Extraordinary and Unusual Adventures of Horatio Lyle
Why did I think her name was Constance Webb? Some mnemonic confusion of Mary Webb (Gone to Earth) and Constance Garnett (translator of Dostoevsky and mother of Bunny) who have nothing in common except for living in the same time period. Is why I need a smart phone, or at least a notebook, because I'm hopeless with both names and mnemonics, and go blank the minute I walk into a book store.
But once established that it's Catherine Webb, I had no little difficulty finding a copy behind a bunch of other books on the lowest shelf of the YA (used-not-new) section at Doug Miller Books. (If I ever win a lottery, one thing I'll do is find Mr Miller larger premises. All those unseen books in boxes and double piles at the back where there's no light frustrate me in the extreme; besides which, if there are three people in the place there's no room to move.)