I'm looking for a children's science fiction book I vaguely remember from around the late 70's in the UK.
From what I remember it features a boy onboard an alien spacecraft where the aliens appear green and robotlike (they and their craft have organic features.) A distinctive feature is their mouth area which appeared like Darth Vader's mouth grill on the front of his helmet.
All I can remember is that the boy hides and an alarm is triggered.
The book was mainly illustrations, and I can't recall the title (hence here.)