Looking for a children's book I remember from the late 70's/early 80's

Question

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.)

Answer 1

An outside possibility - "Stowaways to Venus" (1956) by William Hammond. This was one of the illustrated stories in the 1956 "I Spy Annual". A boy and his sister stow away in a Venusian spacecraft. An alarm is triggered and they are found, but they end up traveling to Venus where invaders from Mercury have attacked using war-robots. I wouldn't really say 'Darth Vader'-like, but but both alien races are very non-human.