So let's talk about Corbyn and Labour and Brexit in a way that is relevant.

Rumour has it that his speech on Monday will declare Labour will seek 'A' customs union, but it cannot be 'THE' customs union.   This seems to me a crucial speech. He may back out and not say anything at all. After this build up, the Tories and Brexiteers would exploit the hell out of that, and it could dismay and lose a lot of his support.

Or, if he overdoes the distinction between 'A', and 'THE', he risks an immediate answer from the EU that this is still "cake and eat it", with a slightly different cake. "Agree where we want, differ where we want? How is that different to Teresa May's stance?" they may say. Again, I could see that being exploited to make a hard Brexit more likely. "You've tried everything else with no success so a complete break is the only option" will be the most pleasant thing they will say.

So the only winning position I see is for the EU response to be "We can take that as a negotiating position. Naturally we disagree, but we can work from there".

It will be a tough speech to pitch correctly.