Complex sentence: The Doctor changes his mind when Turlough reminds him of what the Silurians intend to do if they launch the missiles, and when he is unable to find anything else harmful but less lethal-so begins to connect the gas containers to a central air pump.
Split into simpler sentences: The Doctor changes his mind when Turlough reminds him of what the Silurians intend to do if they launch the missiles. When he is unable to find anything else less lethal, he begins to connect the gas containers to a central air pump.