Fires of Pompeii currently free on iTunes
This is no doubt due to Peter Capaldi's appearance in the episode. They also made Fear Her free around the time of the 2012 London Olympics.
"Human beings worship the great god Santa, a creature with fearsome claws, and his wife Mary. And every Christmas Eve, the people of UK go to war with the country of Turkey. They then eat the Turkey people for Christmas dinner like savages."