SIMPLE ideas are always the best, and French novelist Pierre Boulle came up with a beauty when he created “The Planet of the Apes.”   By reversing the role of men and apes, he created an exciting new science fiction world in which to refresh familiar plots which bear a remarkable resemblance to old-fashioned Westerns.   Being an easy prey to any kind of science fiction, I was enthralled by the first of the new “Planet of the Apes” television series from London Weekend last night.   Prejudices   It is a great novelty to hear all the old human prejudices and fears issue from the lips of the gorillas, the chimpanzees and the orang utans, while the docile human beings serve their simian masters.   When two astronauts get into a tangle with their time warp and crash-land on an ape-ruled earth 2,000 years ahead of their time, one wise old ape urges an impetuous young monkey to keep the evidence of the crashed spaceship secret.   “Humans know their place,” he explains. “If they find out that there are other humans who can make machines like this, they’ll start to think they’re as good as we are.”   What the wiser and older apes know, of course, is that the humans had managed to destroy their civilisation some time in the past by being a bit too clever with their machines and bombs.   All the intelligence has been bred out of them and the apes are determined to keep it like that.   Unfortunately, the two surviving astronauts have brought the old “infection” of knowledge back to the human world and they must be hunted down, brainwashed and killed.