David's father was (John) Felix Cowgill (1903-91), who had the unfortunate distinction of being the MI6 boss of Kim Philby. David's mother was Mary Alexander Wares (1909-99); David was proud of the Scottish ancestry of his mother's family. David never married.
David was educated at Uppingham, then at Edinburgh University.
David taught at The Leas from September 1963 to July 1971; from September 1967 he was assistant headmaster. The Leas had been his second teaching job, after a start in Dalhousie School, Fife, Scotland. In a testimonial, Henry Silcock described David as "the finest schoolmaster" he had ever known.
Before teaching, David had done National Service in the Royal Navy; he stayed on in the Royal Naval Reserve, and would often be seen heading out in his uniform after school.
After leaving The Leas, he taught for a while at Tre Arddur House, Anglesey, then at Buckland House School, Beaworthy, Devon; he seems to have been headmaster there for its last years before it closed in 1984. His last job was as Second Master at Mount House School, Tavistock, Devon, from 1986-98; he suffered a stroke some 6 months before his formal retirement.