Service mechanic

Service mechanics were androids produced by the Kraals which were used to patrol the area around the replica of the 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.

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.

The mechanics' true appearance was only shown once, when Sarah Jane Smith stepped outside the village pub, Fleur-de-Lys, and encountered one of them, with its helmet visor raised, standing near the Ford Transit pickup truck used to transport the android villagers. Beneath the helmet was an electronic 'face' like that of, for example, the Sarah Jane Smith android, except there were no false eyes. (TV: The Android Invasion)

Behind the scenes
The service mechanics were played by Roy Pearce, Keith Ashley, Clinton Morris and Derek Hunt, who remained uncredited both on-screen and in Radio Times.