"Fossilised Fishhooks" sounds totally 1950's even if it shouldn't do. And The Thuggery always come across to me as early 1970's, which is odd since the book was published in 1965, but maybe it's because I was too young to take much notice in the 60's.)

It seems late 50s and early 60s to me - just about the time that the cinema stopped being family entertainment because television was taking over.
As for invented languages - havent the ypung always done that? i was just interested that she wasn't trying to reproduce anything but making it up.