A little harsh to bame this on "the Left" in general. The people who placed that banner are animals, regardless of their political opinion, and no normal person, of any political leaning, would condone such actions.
I agree gulliver - that statement was very much associated with the IRA after the bombing of the Brighton hotel containing the Tory government of the time.
Perhaps Boris and his Cons are not welcome in Manchester, why don't they choose a place up North to hold their Conference. ... Oops sorry I forgot...
They would need petrol to drive there.
It takes one or at most two unknown people of unknown political persuasion to place that banner. Hence it can't be accurately described as "the lovely left".