Waif of the river

One of Jeffery Farnol's vigorous novels of adventures in Georgian England - gravitating from the rurality of Kent and Sussex to the stews and fine houses of London - involving Mr. Jasper Shrig. Plot: mysterious young lady is rescued from the Thames. She then disappears, and a new young lady meets and makes (platonic?) relations with the hero. Great adventures with real peril ensue, the hero falls out with everyone important to him, and dastardly baddies with (of course) foreign overtones do their damnedest.