At the moment, Yougov seems to be completely offline, so I can't check the methodology.
@C R - I'll admit it's hard to reconcile that Graun article (which seems to be the outlier) and other polls on the subject. And quite apart from that, a lot can happen in two years anyway. I'd absolutely agree that it would be nuts to think about a referendum without a pretty unassailable lead. But that's different from going into an election saying you're the party to vote for if you want to improve and formalise EU relations, with a choice of outcomes possible which are short of (re)joining.