Black Hearts in Battersea, by Joan Aiken, 1964

Simon journeys to London to find his friend, Dr. Gabriel Field, under whose direction he intends to enter art school, but the doctor seems to have vanished. He meets the despicable Twites, his landlords, and their irrascible daughter Dido; and rediscovers his good friend Sophy, now lady's maid to the Duchess of Battersea. But a chorus of rascals comes and goes between the Twite home and Battersea Castle, and when Simon discovers an arsenal of weapons in the Twite's cellar, he suspects a Hanoverian plot against Good King James. Rollicking good fun set in an alternate England where the Glorious Revolution never happened, full-fledged danger and villainry that isn't watered down for the kiddies, and a literary original you'll not soon forget, Dido Twite. Most strongly recommended; read The Wolves of Willoughby Chase first.