Doctor Who: The King's Demons: A 5th Doctor Novelisation

It is March 4, 1215, and the TARDIS materializes in England during a jousting match held in the presence of King John. It soon becomes apparent to the Doctor that something is very wrong. Why does John express no fear or surprise at the time travelers' sudden appearance, and indeed welcome them as the King's Demons? And what is the true identity of Sir Gilles, the King's Champion? Very soon the Doctor finds himself involved in a fiendish plan to alter the course of world history, by one of his oldest and deadliest enemies.