As the album title suggests (A Saucerful of Secrets) this track is about ufo sightings. Mildenhall is a us airbase near Rendlesham Forest which had one of the most significant ufo sightings in history. The us government are alleged to have manipulated ufo interest to cover up top secret aircraft projects, hence, a saucerful of secrets.
Hereward the Wake (meaning "watcher") had a base near Ely and roamed around Cambridgeshire. Maybe Roger Waters tapped into "psychic emanations" on the Astral Plane (Collective Unconscious) as floyd had connections with Cambridge, Syd lived there. I've tried correcting the lyrics but these were rejected. The name is Hereward not Heremond. He was an Anglo Saxon leader who resisted the Norman Conquest.