Anyway, I finally got round to searching the web for this book - 15 minutes with Google last night turned up the details, thanks to David Drake for whom it is also an early literary memory. The book is "The Angry Planet" by John Keir Cross. David's description brought it all flooding back:
The Angry Planet is in form a typical children's SF novel of the period. A Scottish scientist builds a spaceship in his back yard. He and a writer-friend fly to Mars in it. Three children in their early teens, staying with the writer, stow away on the spaceship.Even better, thanks to AbeBooks I have tracked down a copy in Leigh on Sea for £5, and I now know that the talented illustrator was Robin Jacques. I wonder what else he did? Time for another Google....
The atmosphere of Mars is breathable. The planet is inhabited by the Beautiful People--willowy, intelligent folk with a fringe of cilia which serve them as hands--who inhabit glassy cities, and their racial enemies, the Terrible Ones--massive, tentacled monsters; also intelligent but utterly evil.
The humans visit the Beautiful People's city; one of the children is captured by the Terrible Ones but escapes; and there's a climactic battle in which the humans aid the Beautiful People against the monstrous foes. The humans return to Earth.
The really strange aspect--for a children's book--is the theme. The Angry Planet is a clear story of the battle between Good and Evil. Evil wins.