Doctor Who and the Dinosaur Invasion

The Doctor walked slowly forward into the cul-de-sac. The giant dinosaur turned its head to focus on the midget now approaching... the Doctor aimed his gun to fire... suddenly from behind came a great roar of anger. He spun round – blocking the exit from the narrow street towered a Tyrannosaurus rex, its savage jaws dripping with blood...

The Doctor and Sarah arrive back in the TARDIS to find London completely deserted – except for the dinosaurs. Has the return of these prehistoric creatures been deliberately planned and, if so, who can be behind it all?