In fact, I think it was Corbyn, given the clothes match ones Corbyn can be seen wearing that day. However, it is NOT a banner Corbyn spoke in front of, as many will have assumed from Mensch's tweet. You can easily find via Google pictures of Corbyn speaking that day 1st May 2014, in which it's clear the offending banner is nowhere in sight (please look here and here).
What's the significance of this? Not very much. Frankly I couldn't care very much if Corbyn had spoken in front of the ludicrous banner. However, he didn't, and in fact it is clear that in the picture he is deep in conversation, that the place largely empty, and that two people are just hand-holding the banner up. The banner was not a rally fixture, even. It's just being waved around by a couple of clowns while Corbyn looks in the other direction.
PS If the picture Mensch tweeted was taken early (and I guess it was - looks like Corbyn has a clipboard and is preparing for the event) then those holding the banner were actually told to clear off! So much for the thought that Corbyn is 'comfortable' about appearing with such banners.