William Herbrand Sackville, 66, the 10th Lord De La Warr, descendant of a British explorer who gave the family name to the state of Delaware. He became the Earl De La Warr on the death of his father, a one-time Conservative Party minister, in 1976. The De La Warrs are among Britain's oldest families, and it was the family name that their ancestors who entered a bay on the American East Coast in 1610 gave to the surrounding land--Delaware, one of the 13 original states. The 10th earl owned England's 6,400-acre Ashdown Forest, which author A. A. Milne made famous as the imaginary playground for Winnie the Pooh. He tried unsuccessfully to sell it last year after an application to drill for oil was rejected. In London on Tuesday after falling under a train.