Cruel Trool? - George Ian Stuart

The super-smart Trebians have been in control of Planet Earth for twenty-five years. The five million citizens are living lives of ease in their Gardens of Delight on an island that used to be called Britain.
With some reluctance, the invaders have allowed a small group of human survivors to begin farming inside a secure enclosure in the Lake District. The idyllic Trebian-controlled weather has allowed the farm to prosper and the humans now number over three hundred. Thousands of Trebian secret cameras watch the humans' everyday lives, including their most intimate moments. These 'reality' shows are the most popular programs on MindVision, sometimes attracting viewing figures of almost 100%.
But then - disaster! The warlike Clonians send a fleet to annihilate the Trebians - and, incidentally, the humans, too! The Trebian Mindminders discover they are powerless against Clonian thought-proof helmets. Only super-mindminder, Si Trool can save her people and she is nowhere to be found.
Si's ability to communicate with deceased minds has given her delusions of God-like grandeur. Will she be too preoccupied to fly to the rescue?

This study of human strengths and weaknesses through the eyes of a brilliant alien race will make you laugh - and think!