Damn. I spent ages composing a post about what I considered to be 'extreme' or 'hard' left and accidently deleted it!
So, very briefly, I associate hard or extreme left with the practice of the ideology devised by communist philosophers/thinkers. Many of the actual principles were absolutely fine, but they took no account of individuals and the leaders of the revolutions which enabled them had very little real idea of how to implement putting them into practice. They ended up resorting to the methods of dictators every where with severe repression of anyone who deviated from the beliefs they imposed on the population. Violent, cruel and inhumane repression.
I don't actually believe that most left wingers in the UK would actually want a revolution, nor would they want non consensual imposition of their beliefs. So I don't see them as a danger or anything to be feared.
I do, however, feel, like Casdon, that change is better implemented incrementally.
Though what seems like a small change to some people can seem like a Revolution to others...
I would observe that revolutions are usually achieved by charismatic demagogues stirring up a large part of the populace. Once they've succeeded in overthrowing the previous regime they really don't have much of a plan for what is to happen next, and discover that it's more difficult to get 'the people' to co-operate when they're not in a state of high emotion whipped up by their demagogic rhetoric. That's when the repression begins...
I have to say that this cycle bears a strong resemblance to our very own Brexit revolution...