Tom and the Dark Knight by Tony Bradman
Tom has the biggest mouth in the kingdom and now he's put his foot right in it! Thanks to him, the terrifying Dark Knight thinks that Tom's father, the tubby, soft-hearted Sir John, is a champion fighter, and he's determined smash him to pieces in Creaky Castle's Grand Jousting Tournament. How can Tom save his father without dropping himself into SERIOUS trouble? Madcap schemes, thumping duels and fearsome foes...it's a medieval knightmare at Creaky Castle!