Service mechanic

Service mechanics were androids produced by the Kraals which were used to patrol the area around the replica village of Devesham and the nearby Space Defence Station on the Kraals' home planet of Oseidon, which served as a training ground for the androids.

They wore white overalls and helmets with dark-tinted visors, and had guns built into the index fingers of their right hands.

History
After the Fourth Doctor and Sarah Jane Smith first encountered and were shot at by the mechanics, the Doctor — who at the time believed they had landed back on present day Earth — wondered if they had wandered into a restricted area and the white overalled figures were some kind of guards. However, Sarah pointed out to him that in England trespassers were prosecuted and not shot.

When Sarah stepped outside the village pub, Fleur-de-Lys, she saw one with its helmet visor raised, standing near the large tradesman's pickup truck used to transport the android villagers. Beneath the helmet was not a face but plastic and electronics. (TV: The Android Invasion)

Behind the scenes

 * The service mechanics were played by Roy Pearce, Keith Ashley, Clinton Morris and Derek Hunt — all of whom remained uncredited both on-screen and in Radio Times. (REF: The Doctor Who Production Guide Volume Three: Cast and Crew)
 * The service mechanics are referred to by this name only once on-screen: in part three, when Styggron is heard over a loudspeaker in the Kraal base to order them to the rocket's loading bay. The android of Sarah calls them "robot mechanics" in part two.
 * In Terrance Dicks's novelisation of the story, there is just empty space beneath the helmet of the service mechanic Sarah sees outside the pub.