Thirty years ago, when the West was celebrating the collapse of the Berlin Wall, one could have added to the merriment by suggesting that in 2019 Britain would find itself  dependent on the whim of a tiny proportion of voters to avoid succumbing to Marxist government. Tee-hee – what a caution: that would have been the universal response to any implausible Cassandra who forecast then the menace that overshadows us now.