Bodkin became a ghost during the middle 1500s, Fanny the middle 1600's, Matt the 1700's, and Sir George the 1800's. The white lady doesn't know when she's from but she's been around longer than anybody except maybe Old Glory, who has been there since before Motley Hall was built.
Technically, Sir Humphrey Uproar was not actually the last of the Uproar clan. We meet his sister, who Mr. Gudgin refers to only as "Miss Uproar" in the episode entitled "The Christmas Spirit", S2, ep.1. Following the death of her brother, she is visiting the house one final time before returning to Thailand and entrusting Mr. Gudgin to oversee the sale of Motley Hall.