Jeremy Corbyn is a rarity among politicians. All his enemies are on his own side. For the Tories, Ukip and the SNP, Corbyn is a dream made real. They could not love him more. As the riotous scenes at the shadow cabinet and parliamentary Labour party meetings this week showed, his colleagues see Corbyn and John McDonnell as modern Leninists who are mobilising their cadres to purge all dissidents from the party.