Edward the Confessor

 was King of England in the 11th century, one of the last Anglo-Saxon kings before the Norman Invasion. Both he and his Queen Edith of Wessex were considered shrewd rulers during a time of peace. The Doctor visited his court in 1055 and exposed a plot against the crown by Leofric of Essex. (BFA: Seasons of Fear)

Biography
When Edward was a child, the Doctor had visited the court of his father, Æthelred the Unready, and cured Æthelred of a heavy fever. (BFA: Seasons of Fear)

In 1055 Bishop Leofric of Exeter presented the King and Queen with exquisitely crafted necklaces, made of a new metal that was warm to the touch. The Doctor arrived shortly afterward, to Edward's pleasure... and Edith's chagrin. The Doctor's interest (and Edith's suspicions) made the two of them contrive a ruse to get Leofric to expose his plan: kill the King and Queen with neckalces made of plutonium, and assault the castle with tons of the stuff in an attempt to summon the Nimon to Earth. With the Doctor's help, they defended the castle from attack, and disposed of the plutonium. The Doctor gave them anti-radiation pills to counteract the effects of the necklaces and assault.