The Cuckoo Tree — Joan Aiken

When Dido Twite sets foot back on English soil, more mischief awaits. As her friend Captain Hughes recovers from a carriage accident, Dido stays at the Dogkennel Cottages and meets the odd inhabitants of Tegleaze Manor: strange old Lady Tegleaze, her nephew, Tobit, and his wizened, witchy nurse, Sannie. Soon suspicious things happen. A priceless miniature is stolen. Tobit is framed and then kidnapped. A twin sister is found. And when Dido catches a glimpse of her rascally father in Petworth, she is sure she's in the midst of another Hanoverian plot. Can she get to London to warn the king and save St. Paul's Cathedral from sliding into the Thames?