So that means, if Corbyn denies the allegation, its up to him to prove it?
Or the papers to prove they are accurate?
It would be up to the Papers to prove it's true but Corbyn would have to show proof that's it's caused him reputational and or monetary harm.
They'd have three main defences: Fair comment - usually an opinion which wouldn't count here, privilege - knowing of/being subject to information and justification - similar to fair comment but a good solicitor could argue it and then the truth of the matter/is the story in public interest - see the last one.
Corbyn wouldn't chase the papers for it because it makes him look like a dictator, in a way, and leaves him 'trapped' in the sense that he'd be doing himself more reputational harm than good by chasing what he sees as a senseless story.
If, say for example, it was a more defamatory story then he'd probably chase it but it makes sense to use it as fire going forward to take on the press and push IPSO into doing real action instead of just being wheeled out every so often to so 'yeah, that's bad' or 'no, that's ok' like they do now.