So Vile A Sin

"Of course the Martians own Earth, we surrendered in 2010" 

When the Seventh Doctor comes into direct contact with the Nexus, a primary source of unimaginable psi power, it releases every potential possibility of his existence. The probabilities spray loose into the galaxy, some of them finding places to settle. One of the probabilities settles on Earth. In this timeline the Doctor surrendered the Earth to the Ice Warriors in 2010. The Martian Empire protected the Earth from attacks by the likes of the Daleks and the Rutans. The human race lives in a Utopia, in harmony with the planet Earth, which has become a nature reserve. Meanwhile, the Doctor has grown to a great age in a single body and lives a small cottage in the countryside, with an Earth Reptile as a companion.
(pages 138-149) 

Later, Roz Forrester looks for a man called Zatopek in the Martian quarter of the world of Cassandra. As Roz sits in a cafe, an Ice Warrior sits down at her table and warns that she should discontinue her search. It turns out the Martian is a former Adjudicator. Before he leaves the Ice Warrior gives Roz this last piece of advice: 

"I also advise you to avoid the square, blue-coloured cakes," he said. 
"Why?" Roz glanced down at her plate. "Are they poisoned?" 
"No," said the Martian, getting up. "But they're terribly fattening." 
(pages 183-185)